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Materials Chemistry and Physics 115 (2009) 612–617

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

agic behavior and bonding nature in hydrogenated aluminumhosphide clusters

ing Guo ∗

chool of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China

r t i c l e i n f o

rticle history:eceived 7 March 2007eceived in revised form 30 December 2008

a b s t r a c t

Interaction of hydrogen with aluminum phosphide clusters has been investigated using the density func-tional method of Becke’s three-parameter hybrid functional with the nonlocal correlation of Lee, Yang, and

ccepted 23 January 2009

eywords:agic behavior

onding natureluminum phosphide cluster

Parr. Berny structural optimization and frequency analyses are performed with the basis of 6–311 + G(d).Our results show large binding energies of a single hydrogen atom on small AlP clusters and large highestoccupied and lowest unoccupied molecular–orbital gaps for (AlP)H and (AlP)5H making these speciesbehave like magic clusters. Calculations on two hydrogen atoms on AlP clusters show large binding ener-gies for (AlP)nH2 with n = 1, 3, 5, and 7. In general the binding energy of H and 2H are both found todecrease with an increase in the cluster size. And the calculations also suggest that hydrogen should be

(AlP)

ensity functional theory dissociated on (AlP)2 and

. Introduction

The chemistry and physics of the compounds formed by thelements in groups III and V is extraordinarily rich and their use-ulness in the semiconductor industries has been a motivation forhe numerous experimental and theoretical studies [1–6]. Amonghem, the aluminum phosphides have received considerable atten-ion, as they have higher vibrational frequencies (due to lower

asses), and, thus, as noted by Gomez et al. [7], could result inibrational progressions in the spectra compared to heavier clus-ers. In addition, the smaller number of electrons makes them moremenable to electronic structure calculations. There have beenome previous theoretical studies on (AlP)n cluster. Raghavacharind co-workers [8] calculated minimum-energy structures forAlP)n using Hartree-Fock (HF) and fourth-order Moller-PlessetMP4) perturbation theory, followed by quadratic configurationnteraction QCISD(T). On the basis of local density functional cal-ulations, Tomasulo and Ramakrishna [9] analyzed the structureor (AlP)n (n = 1–6) clusters, finding significantly different structureshan those for Si2n cluster, even though AlP and Si2 are isoelectronicnd the corresponding bulk materials have the similar lattice andand structures. Using the complete active-space MCSCF (CASSCF)

ollowed by multireference singles + doubles configuration inter-cbtion (MRSDCI), Feng and Balasubramanian [10] studied theround state and electronic states of (AlP)2 and its ions and foundhat the 1Ag, 2B1u, and 2B1g electronic states with rhombus equilib-

∗ Tel.: +863572051192; fax: +863572051192.E-mail address: gl-guoling@163.com.

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.01.025

3.© 2009 Elsevier B.V. All rights reserved.

rium structures were found to be the ground state of (AlP)2, (AlP)2+,

and (AlP)2−, respectively. Archibong et al. [11] calculated structures

and detachment energies for Al2P2− and its neutral counterparts at

the DFT and coupled cluster singles and doubles (CCSD (T)) levels oftheory. They reported ground and excited-state electronic energiesand vibrational frequencies for neutral and anionic species. Archi-bong’s work differs from those of Balasubramanian in that Al2P2was not restricted to D2h (Planar rhombic) symmetry, and indeed,Al2P2

− was found to have a nonplanar ground state. Costales et al.[12] have also theoretically investigated the structure, stability, andvibrational properties of the (AlP)n (n = 1–3) using both Gradient-corrected (GGA) Becke exchange functional [13] and Wang andPedrew [14] correlation functional. They observed that aluminumphosphide monomers, dimmers, and trimers exhibited the samebehavior as the aluminum nitride clusters. Recently, Karamanis andLeszczynski [15] reported the correlations between bonding, size,and second hyperpolarizability of small semiconductor cluster: Abinitio study on AlnPn clusters with n = 2, 3, 4, 6 and 9. Oncak andSmec [16] calculate the stability and properties of AlP monomers,dimers and trimers using ab initio method. Zhao et al. [17] explorethe lowest-energy structures of AlnPn clusters up to n = 9 usingall-electron density functional calculations with a gradient correc-tion.

Interaction of hydrogen with III–V compound semiconductorshas attracted much attention over the past decades. An early effort

to understand hydrogen interaction with phosphorus-rich indiumphosphide and gallium-rich gallium arsenide has been made by Fuet al. [18] and Schailey and Ray [19]. However, understanding itsinteraction with clusters is still primitive even though experimen-tal studies have been available for quit some time. This is primarily

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L. Guo / Materials Chemistry

ue to the fact that the understanding of the atomic and electronictructures of clusters has itself been a major problem. Studies ofnteraction of atoms and molecules with clusters are important foratalysis as well as for the development of cluster-based materi-ls. Hydrogen is very special, as it constitutes an integral part ofeveral organic materials that are technologically important withegard to catalytic reactions and also studies of biological systems.urther, there is great interest in hydrogen interaction with clus-ers could lead to novel hydrogen-absorbing nanomaterials [20].lthough no effort to study hydrogen interaction with aluminumhosphide clusters at present, understanding of relativities of alu-inum phosphide clusters could give useful insight on hydrogen

nteraction with III–V clusters.In the following, we present results of first-principles total-

nergy of calculations on small aluminum phosphide clusters with–8 and 12 AlP and up to two hydrogen atoms. These provide andnderstanding of the nature of interaction of hydrogen with alu-inum phosphide clusters and the magic behavior of these clusters.

n Section 2, we give the details of our computational method. Theesults are presented in Section 3. Section 4 contains conclusionsf the paper.

. Methodology

Structural optimizations in this work have been performed using the densityunctional theory of Becke’s three-parameter hybrid exchange functional [21] withhe LYP correlation functional (B3LYP). Frequency analyses at the optimized struc-ures are carried out at the same theoretical level to clarify if the optimized structuresre true minima or transition states on the potential energy surfaces of specificlusters. All of the obtained most stable clusters are characterized as energy min-ma without imaginary frequencies. Further studies of the stability of clusters haveeen carried out using the Gaussian method with a Perdew–Wang 1991 exchangeorrelation [22]. We carried the calculations out for spin multiplicities of 2S + 1 = 1nd 2S + 1 = 2 for clusters with even and odd numbers of electrons, respectively.ll calculations are carried out using the Gaussian 98 program [23] on SGI/O2orkstations in our laboratory. The geometries are fully optimized. Our results of

tructures of small (AlP)n (n = 1–6, 12) clusters agree well with those reported in theiterature [9,24,25]. Those of (AlP)7 and (AlP)8 are optimized for the first time byurselves.

. Results and discussions

.1. Hydrogen on (AlP)1–5

The results of the atomic structure of one and two H atomsn small (AlP)n clusters are shown in Fig. 1 while, the calcu-ated HOMO-LUMO energy gaps, binding energies and the meanearest-neighbor bond lengths of (AlP)n (n = 1–8, 12) and (AlP)nHm

n = 1–8,12; m = 1,2) clusters are given in Tables 1 and 2, respectively.t is seen that the BE of H on AlPH (3.521 eV) is close to that of H onlP (3.383 eV). This shows that both AlPH and AlPH2 have the sametability.

The BE of H on (AlP)2 is 2.571 eV. The C1 isomer [Fig. 1(2c)] withwo H atoms in the Al and P atom and the C2v form [Fig. 1(2d)] withwo H atoms in the two Al atoms are two energetically degeneratetructures. Their energy distance is only by 0.007 eV.

(AlP)3 is a planar benzene-like structure [Fig. 1(3a)] with D3hymmetry. Interaction of H on (AlP)3 is favorable on a low coordi-ation (onefold) site of P [Fig. 1(3b)] as compared to a onefold sitef Al [Fig. 1(3c)] by 0.29 eV. The BE is small (2.439 eV) that it is likelyake further interaction with hydrogen atom. In order to confirm

his we carried out calculation on (AlP)3H2. We studied three con-gurations for H: (i) where one H is on the top site of P atom andnother on the top site of the neighboring Al atom [Fig. 1(3d)], (ii)

here two H atoms are on top sites of two opposite Al and P atoms

Fig. 1(3e)], and (iii) where two H atoms are on top sites of twoeighboring P atoms [Fig. 1(3f)] making the Cs structure as shown

n Fig. 1(3d)–(3f). The energy difference of the first two of theses 0.44 eV (Table 2). Also the Cs structure 3f lies 0.55 eV higher in

hysics 115 (2009) 612–617 613

energy than the 3d structure. The BE for 2H is 5.561 eV (Table 2)and it shows that interaction between two hydrogens on (AlP)3 isattractive. This energy is higher than the dissociation energy of H2(4.6 eV). Accordingly, hydrogen is likely to be dissociated on (AlP)3.The distance between two hydrogens on (AlP)3 in the lowest-energystate is 3.35 Å as compared to the bond length of 0.75 Å in H2.Therefore, two hydrogens are in a dissociated configuration. Thedissociation can happen on a top site of (AlP)3. Since there are sev-eral such sites, the probability for such a dissociative process is alsohigh.

(AlP)4 has a planar D2d structure [Fig. 1(4a)] and H, similar tosmall clusters above, prefers a top site of P atom [Fig. 1(4b)] on it.In the case of two H on (AlP)4, one is on the top site of Al atom andthe other, on the top site of neighboring P atom [Fig. 1(4c)]. Theneighbor site of two P atoms [Fig. 1(4d)] is 0.39 eV higher in energy.It is the 3D structure with C2v symmetry. The structure 4e is 0.57 eVhigher than 4c. Its one H atom is on the top site of P atom and theother H atom, on the bridge site of neighboring Al and P atoms.

Similar to (AlP)4, (AlP)5 is also a 3D structure [Fig. 1(5a)] withC1 symmetry. This structure can be viewed as adding one Al atomand one P atom at the same side of (AlP)4. One H is most favorableon a top site of P atom [Fig. 1(5b)], just like the clusters discussedabove. The BE (2.776 eV) of H on (AlP)5 is also one of the largestamong all the clusters we have studied. Accordingly, (AlP)5H shouldhave large abundance. Two H favor top sites of neighbor Al andP atoms [Fig. 1(5c)]. The BE of this isomer is 5.955 eV which isagain quite large and slightly lower than the value for (AlP)H2.This should also make hydrogen dissociate on this cluster unlessthere is a barrier. Isomers with two H on different top sites of Aland P atoms or P atoms [Fig. 1(5d–5g)] have 0.36–1.22 eV higherenergies.

For these small clusters the BE per H is high with n = 1 and 5.And addition of a second H increases nearly the same value of BEfor n = 2–5. On the other hand, for n = 1, addition of a second Hincreases the BE significantly. Our calculations suggest that H2 islikely to be combine at least on AlP small cluster and these clusterscould disintegrate, such as (AlP)H2, or combine with others to formenergetically more favorable species.

3.2. Hydrogen on (AlP)6

The lowest-energy isomer of (AlP)6 is a hexa-prism structure[Fig. 1(6a)] with D3d symmetry. We consider adsorption of singlehydrogen on a top site of P atom [Fig. 1(6b)]. The BE (1.911 eV) ofH on (AlP)6 is the smallest among all the clusters we have stud-ied. The fragmentation energy (see below) is also small and thisgives further support for the instability of (AlP)6H. Accordingly, itmay not have large abundances. For two hydrogen atoms on (AlP)6,several configurations are studied. These include two neighbor-ing top of Al and P atoms in the different hexagon [Fig. 1(6c and6d)] and two neighboring top of P atoms in the same and differ-ent hexagon [Fig. 1(6e and 6f)]. The calculated BEs are given inTable 2. The most favorable adsorption sites are two neighboringtop of Al and P atoms in the same hexagon [Fig. 1(6c)]. The twoH have a similar configuration as in (AlP)2H2. The BE for 2H is4.984 eV and it shows that interaction between two hydrogen on(AlP)6 is not more attractive than clusters discussed above. How-ever, this energy is also higher than the dissociation energy ofH2 (4.6 eV). Accordingly, hydrogen is likely to be dissociated on(AlP)6. The distance between two hydrogens on (AlP)6 in the lowest-energy state is 3.91 Å as compared to the bond length of 0.75 Å

in H2. Therefore, two hydrogen are in a dissociated on (AlP)6. Thedissociation can happen on a top site of (AlP)6. Since there are sev-eral such sites, the probability for such a dissociative process isalso high. Further calculations using the Gaussian method with aPerdew–Wang 1991 exchange correlation gave a 4.794-eV BE for

614 L. Guo / Materials Chemistry and Physics 115 (2009) 612–617

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Fig. 1. Relaxed structures of (AlP)nHm (n = 1–8 and 12; m = 1, 2

wo H on (AlP)6, in close agreement with the result obtained fromhe B3LYP method.

.3. Hydrogen on (AlP)7

For (AlP)7, the lowest-energy structure is a C3v structureFig. 1(7a)]. One hydrogen adsorption is favorable on the top ofhe middle P atom [Fig. 1(7b)]. The BE (2.106 eV) of H on (AlP)7s also one of the smallest among all the clusters we have studied.

y, black and white balls are used for Al, P and H, respectively.

Isomers [Fig. 1(7c) and (7d)] with H on the top site of capping Patom in the upper part of the cluster [Fig. 1(7c)] and the top siteof P atom in the bottom [Fig. 1(7d)] are 0.14 and 0.28 eV higher inenergies, respectively. The small HOMO-LUMO gap is likely to make

further interaction of hydrogen with this cluster energetically notso favorable. In order to confirm this we carried out calculationson (AlP)7H2. Several initial configurations were considered for twohydrogens. These include H atoms on the top of two neighboringAl and P atoms in the lower part of the cluster [Fig. 1(7e)]. This

L. Guo / Materials Chemistry and P

Table 1Binding energies (BEs) per AlP molecule, and HOMO-LUMO gaps of various AlPclusters obtained using the B3LYP-DFT method.

Cluster Gap (eV) BE per AlP molecule dAl−P (Å)

1a AlP 3.99 3.853 2.232a (AlP)2 2.25 6.097 2.543a (AlP)3 2.16 6.622 2.234a (AlP)4 1.89 7.038 2.385a (AlP)5 1.58 7.163 2.356a (AlP)6 2.35 7.561 2.367a (AlP)7 2.36 7.592 2.448a (AlP)8 2.73 7.768 2.42

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atoms [Fig. 1(8e)] and two opposite top sites of neighboring P atoms

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12a (AlP)12 3.38 8.034 2.32

Al–P is the mean nearest-neighbor bond lengths between Al and P atoms.

as the lowest energy. The HOMO-LUMO gap is lower (2.41 eV)nd the addition of one more hydrogen to (AlP)7 leads to a gainf 3.062 eV, an increase of more than 0.96 eV in the BE of H as com-ared to one hydrogen on (AlP)7. The other calculated positions forwo hydrogen on (AlP) are one on top of the capping P atom and

7ne on the neighboring Al atom [Fig. 1(7f)], and H atoms on theop of two neighboring P atoms in the bottom of the (AlP)7 clus-er [Fig. 1(7g)], and two opposite P atoms [Fig. 1(7h)]. The energy,he HOMO-LUMO gap, and other structural information are given

able 2inding energies (BEs), structures, and HOMO-LUMO gaps of various clusters obtained uond lengths between Al and P atoms, Al and H atoms, P and H atoms. Location of H is repnd top site, respectively.

Cluster Location of H Gap (eV) Total BE (e

1b AlPH t 3.66 7.2361c AlPH2 n,t 3.32 10.7572b (AlP)2H t 2.75 14.7652c (AlP)2H2 n,t 2.82 17.9572d (AlP)2H2 o,t 3.51 17.9503b (AlP)3H t(P) 2.58 22.3053c (AlP)3H t(Al) 3.29 22.0113d (AlP)3H2 n,t(Al, P) 2.75 25.4273e (AlP)3H2 o,t(Al, P) 2.83 24.9833f (AlP)3H2 n,t(P, P) 2.35 24.8804b (AlP)4H t(P) 2.72 30.7144c (AlP)4H2 n,t(Al, P) 2.48 33.6234d (AlP)4H2 n,t(P, P) 2.42 33.2284e (AlP)4H2 b,t(Al, P) 2.19 33.0515b (AlP)5H t(P) 3.00 38.5915c (AlP)5H2 o,t(Al, P) 2.76 41.7705d (AlP)5H2 n,t(Al, P) 2.22 41.3945e (AlP)5H2 n,t(Al, P) 2.13 41.0575f (AlP)5H2 n,t(Al, P) 2.19 40.8835g (AlP)5H2 n,t(P, P) 1.97 40.5536b (AlP)6H t 2.18 47.2776c (AlP)6H2 n,t(Al, P) 2.50 50.3506d (AlP)6H2 n,t(Al, P) 2.63 50.2276e (AlP)6H2 n,t(P, P) 1.97 49.5016f (AlP)6H2 n,t(P, P) 2.29 48.9787b (AlP)7H t 2.50 55.2507c (AlP)7H t 2.44 55.1037d (AlP)7H t 1.50 54.9677e (AlP)7H2 n,t(Al, P) 2.14 58.3127f (AlP)7H2 n,t(Al, P) 2.72 58.1737g (AlP)7H2 n,t(P, P) 1.88 57.2787h (AlP)7H2 o,t(P, P) 1.58 57.0878b (AlP)8H t 2.66 63.8638c (AlP)8H t 1.64 63.7928d (AlP)8H2 n,t(Al, P) 2.80 67.0368e (AlP)8H2 n,t(P, P) 1.72 65.6298f (AlP)8H2 n,t(P, P) 1.60 65.5882b (AlP)12H t 3.14 97.730

12c (AlP)12H2 n,t(Al, P) 2.82 101.0092d (AlP)12H2 (Al, P) 2.43 100.443

12e (AlP)12H2 o,t(Al, P) 2.54 100.3862d (AlP)12H2 f,t(Al, P) 2.27 100.218

hysics 115 (2009) 612–617 615

in Table 2. The energies of the isomers [Fig. 1(7f-7h)] are close tothat of Fig. 1(7e), and their energy differences with 7e are 0.14, 1.03,and 1.23 eV, respectively.

3.4. Hydrogen on (AlP)8

(AlP)8 has a S4 symmetry [Fig. 1(8a)]. We can very roughlydecompose this structure into two interacting entities: structure(AlP)6 and (AlP)2 are bridged with four Al–P bonds. We anticipatethat, similar to (AlP)6, this cluster would favor to react with onehydrogen. Indeed we find that, the BE of H is 1.719 eV similar to(AlP)6 of 1.911 eV. One H is favorable on a top site of P atom in thebottom part of (AlP)8 [Fig. 1(8b)]. Structure Fig. 1(8c) with H atomon a top site of middle P atom is only 0.071 eV less stable. There-fore, the interaction depends very sensitively on the electronic andatomic structures of clusters. We studied adsorption of two hydro-gen on a few selected sites which included the two H atoms on thetop sites of neighboring P and Al atoms in the upper part of (AlP)8[Fig. 1(8d)], two neighboring faces with H atoms on the different P

in the upper part of (AlP)8 [Fig. 1(8f)]. We find that the 8d isomerhas the lowest energy (Table 2). The BE of this isomer is 4.892 eVwhich again quite small and similar to (AlP)6H2 and only slightlyhigher than the value for (AlP)12H2.

sing the B3LYP-DFT method. dAl–P, dAl–H, and dP–H, are the mean nearest-neighborresented by symbols n, o, b, f, t which mean neighboring, opposite, bridge, farthest,

V) BE of H (eV) dAl–P (Å) dAl–H (Å) dP–H (Å)

3.383 2.22 1.446.904 2.44 1.57 1.432.571 2.32 1.435.763 2.32 1.58 1.435.756 2.28 1.572.439 2.26 1.432.145 2.26 1.585.561 2.27 1.58 1.435.117 2.28 1.59 1.435.013 2.31 1.422.562 2.38 1.415.471 2.39 1.58 1.415.076 2.22 1.414.899 2.18 2.06 1.462.776 2.37 1.435.955 2.35 1.57 1.435.579 2.38 1.57 1.415.242 2.36 1.58 1.415.068 2.38 1.424.738 2.37 1.411.911 2.36 1.414.984 2.37 1.58 1.424.861 2.36 1.58 1.424.135 2.37 1.413.612 2.37 1.432.106 2.36 1.411.959 2.36 1.411.823 2.36 1.415.168 2.36 1.58 1.425.029 2.36 1.58 1.414.134 2.35 1.433.943 2.36 1.411.719 2.36 1.431.648 2.35 1.414.892 2.35 1.58 1.423.485 2.36 1.413.444 2.36 1.411.322 2.33 1.434.598 2.33 1.58 1.424.035 2.34 1.58 1.413.978 2.36 1.58 1.413.810 2.37 1.58 1.41

616 L. Guo / Materials Chemistry and Physics 115 (2009) 612–617

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Fig. 2. Binding energies of H (left)

.5. Hydrogen on (AlP)12

For (AlP)12, the Th symmetrical (AlP)12 cluster [Fig. 1(12a)] haseen computed to be the most stable using B3LYP/6-311 + G(d)ethod which is in good agreement with ref. 25 optimized by Ioan

t al. We consider adsorption of single hydrogen on the top sitef P atom [Fig. 1(12b)]. The BE (1.322 eV) of H on (AlP)12 is one ofhe smallest among all the clusters we have studied. However theOMO-LUMO gap is large (3.14 eV). The large HOMO-LUMO gap

s likely to make further interaction of hydrogen with this clusternergetically no so favorable. In order to confirm this we carriedut calculations on (AlP)12H2. Several initial configurations wereonsidered for two hydrogens. These include two H atoms on theop sites of neighboring and opposite Al and P atoms in the sameexagon [Fig. 1(12c) and 1(12e)] and two H atoms on the top sites ofl and P atoms in the neighboring faces [Fig. 1(12d) and 1(12f)]. The(12d) isomer is 0.57 eV less stable as compared to the 1(12c) iso-er that is the most favorable. Isomers 1(12e) and 1(12f) are almost

egenerate which are 0.62 and 0.79 eV higher in energies than(12c), respectively. In order to further check the results obtainedrom the B3LYP method, we calculated BEs for H on the top sites ofAlP)12 using the PW91 method. It is found that the BEs of hydrogens 1.358 eV. This result is quite close to the value (1.322 eV) obtainedrom the B3LYP method.

.6. Stability and fragmentation behavior

In order to check the stability of the lowest-energy isomers we

alculated the vibrational frequencies for selected clusters using the3LYP/6-311 + G(d) level of theory. It is found that the lowest-energy

somers of all kinds of clusters discussed above have all real frequen-ies and are, therefore, stable. Fig. 2 shows the plot of the BE of onend two H atoms on aluminum phosphide clusters. As discussed in

able 3ragmentation energies of (AlP)nHm clusters with the product (AlP)n−pHm−q , p = 1 and 2, a

Cluster [(AlP)nHm] (AlP)n−1Hm + AlP

2b) (AlP)2H 3.682c) (AlP)2H2 3.353b) (AlP)3H 3.693d) (AlP)3H2 3.624b) (AlP)4H 4.564c) (AlP)4H2 4.345b) (AlP)5H 4.025c) (AlP)5H2 4.296b) (AlP)6H 3.836c) (AlP)6H2 4.737b) (AlP)7H 4.127c) (AlP)7H2 4.118b) (AlP)8H 4.768c) (AlP)8H2 4.87

ll the values mean the parent cluster has a larger binding energy than the sum of the BE

H (right) atoms on (AlP)n clusters.

the previous section, the BE is large for H with n = 1 and 5 of (AlP)n

atoms. However, for 2H, clusters with an odd number of n havehigher BEs. The stability of these complexes is further studied fromthe fragmentation energies (Table 3). We have studied channels inwhich an AlP, (AlP)2, or AlPH molecule is one of the fragments. It isnoted that in all these processes, the fragmentation energy is thelargest for (AlP)8H2 and therefore, we expect it to be among themost stable species. Also the fragmentation energies for (AlP)5H2,(AlP)6H2, and (AlP)7H2 are next largest for all these channels, sug-gesting them to be another stable clusters. On the other hand thefragmentation energy for (AlP)3H is 2.88 eV for (AlP)n−2Hm + (AlP)2and (AlP)n−1Hm−1 + AlPH channels and they are close to the lowestvalues. The small clusters of (AlP)2H also have lower values. Thisanalysis is not done for (AlP)12H which is not among the most stablespecies.

3.7. Bonding nature

From Fig. 1, we find that (AlP)n cluster with direct P–P and Al-Alconnections are less stable, and the most stable aluminum phos-phide cages [i.e. (AlP)6, (AlP)7, (AlP)8 and (AlP)12] have only four-(f4)and six-membered (f6) rings, the number of f4 is always equal to6, whereas the number of f6 is n-4. In order to understand theabove results and the bonding nature of hydrogen on aluminumphosphide clusters, we discuss the bond lengths in both hydro-genated clusters and pure aluminum phosphide clusters. FromTables 1 and 2, one finds that with the exception of AlP and (AlP)2,the Al–P bond lengths increase generally as the size of the clus-

ter increases, and in the range of n = 6–12 clusters, the variation issmall such that all the vertex-vertex distances are within about 0.1%of 2.35 Å. In general there is a slight increase in the mean Al–P bondlengths after H adsorption on the lowest-energy sites. The Al–H andP–H bond lengths for the top adsorption on (AlP)n clusters are in

nd q = 1.

(AlP)n−2Hm + (AlP)2 (AlP)n−1Hm−1 + AlPH

2.57 3.685.76 3.492.88 2.882.48 3.433.76 3.613.47 4.084.09 3.204.15 3.823.37 3.234.53 4.524.46 2.644.35 3.804.39 3.494.49 4.55

of the products.

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L. Guo / Materials Chemistry

he range of 1.57–2.06 Å and 1.41–1.46 Å, respectively. Thus, one canonclude that these bond lengths evolve very slowly with clusterize. In addition, the nearly constant value for P–H and Al–H bondengths on different clusters at specific adsorption sites suggests theimilar nature of bonding of H in different clusters. From Table 2 andig. 1, we can also see that hydrogen would like to be onefold withhe (AlP)n clusters. There is an elongation of the bond length withn increase in the coordination. This is confirmed by longer Al–Pnd P–P bond lengths (2.06 and 1.46 Å) on the bridge site of theAlP)4H2 4e isomer (Table 2).

. Summary

In summary, we have presented results of studies on hydro-en interaction with aluminum phosphide clusters. We find thatydrogen interacts strongly with aluminum phosphide clusters.ur results show large binding energies of a single hydrogen atomn small AlP clusters and large highest occupied and lowest unoccu-ied molecular–orbital gaps for (AlP)H and (AlP)5H making thesepecies behave like magic clusters. Calculations on two hydrogentoms on AlP clusters show large binding energies for (AlP)nH2 with= 1, 3, 5, and 7. In general the binding energy of H and 2H are both

ound to decrease with an increase in the cluster size. And the calcu-ations also suggest that hydrogen should be dissociated on (AlP)2nd (AlP)3.

cknowledgments

This work was financially supported by the National Naturalcience Foundation of China (Grant No. 20603021), Youth Foun-

[

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hysics 115 (2009) 612–617 617

dation of Shanxi (2007021009) and the Youth Academic Leader ofShanxi.

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