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nasbs@xmath0 is a little studied semiconductor that was recently shown to be remarkably effective as a solar absorber material , @xcite and has also been mentioned as a potential thermoelectric material based on computational screening .
@xcite remarkably , an efficiency of 3.18% was obtained in the first report using nasbs@xmath0 nanparticles in a dye sensitized solar cell .
@xcite this is comparable to the efficiency of early cells of similar type made using organometallic halide perovskites .
@xcite those materials have since proven to be a revolutionary advance in photovoltaics , but suffer from issues with long term stability and the fact that they contain pb , which is undesirable .
@xcite nasbs@xmath0 contains only environmentally friendly low cost elements , and this very promising early experimental result , and the theoretical results below suggest that it may as well represent a revolutionary new material for solar photovoltaic applications .    from a valence point of view
one might regard the stoichiometry as derived from pbs by splitting of the divalent pb site into monovalent na and trivalent sb . in this way one might anticipate that nasbs@xmath0 would be a semiconductor and that the band gap may be higher than that of pbs if normal trends are followed due to the splitting of the cation site .
however , at ambient temperature the crystal structure deviates strongly the rocksalt structure of pbs , as discussed below .
more significantly , the presence of na suggests a propensity for defects , e.g. na off - stoichiometry .
good charge collection requires a high carrier drift length , which typically occurs in high quality defect free material , and so the high performance of nasbs@xmath0 as a solar absorber is surprising .
however , several soft lattice solar materials have been discovered , most notably ch@xmath4nh@xmath4pbi@xmath4 , @xcite where defects do not seem to play the same detrimental role as in more traditional materials such as cdte .
@xcite here we report first principles calculations aimed at understanding the properties of this compound , especially in relation to its use as a solar absorber .
the present calculations were performed within density functional theory ( dft ) .
the electronic structure and optical properties were calculated using the general potential linearized augmented planewave ( lapw ) method , @xcite as implemented in the wien2k code .
@xcite the total energy calculations and relaxation of the atomic coordinates was done using the perdew , burke and ernzerhof ( pbe ) generalized gradient approximation ( gga ) .
@xcite for this , relativity was treated at a scalar relativistic level for the valence states .
the core states were treated relativistically .
lapw sphere radii of @xmath5=2.2 bohr were used for all elements , along with a planewave sector basis cutoff determined by , @xmath6=9 ( here @xmath5 is the radius of the smallest lapw sphere , i.e. 2.2 bohr , and @xmath7 is the planewave cutoff ) .
local orbitals were added to the basis for the s @xmath8 , na @xmath8 and @xmath2 , and sb @xmath9 semicore states .
spin - orbit was included for the electronic and optical properties .
the band gap is important for these , and accordingly these calculations were done using the modified becke - johnson ( mbj ) potential of tran and blaha .
@xcite this functional gives band gaps in good accord with experiment for a wide variety of simple semiconductors and insulators and also appears to give reliable band shapes and optical properties , although at least in certain semiconductors the band masses are more similar to those obtained in standard density functional calculations than those from many body calculations .
@xcite calculation of the transport function for conductivity was done using the boltztrap code .
@xcite optical properties were calculated based on electric dipole transitions in the independent particle approximation as implemented in the wien2k code . the born effective charges and the dielectric tensor
were calculated using the density functional perturbation theory ( dfpt ) with the pbe functional as implemented in the vasp code @xcite ( note that calculation of the dielectric tensor can not be done with the mbj potential , since it is not an energy functional and therefore can not be used to evaluate lattice response ) .
the structure of nasbs@xmath0 has been refined into two different monoclinic groups , @xmath10 ( # 15 ) @xcite and @xmath11 ( # 12 ) .
@xcite in addition there is a report of a triclinic structure @xmath12 ( # 2 ) .
@xcite these are all centrosymmetric structures .
the deviation from a cubic structure was discussed in terms of lone pair activity of sb
we did calculations for the three reported structures , in each case using the lattice parameters from the diffraction experiments , and then relaxing the atomic positions subject to the spacegroup symmetry .
we find that the energy for the @xmath11 structure is 0.127 ev per formula unit ( f.u . ) above the energy of the @xmath12 structure .
the @xmath10 structure is 0.005 ev / f.u .
higher than the @xmath12 structure .
we did further calculations to address the issue of the ground state . specifically , we did full relaxations , including both lattice parameters , angles and internal coordinates . for this purpose
we used the vasp code with three different density functionals , specifically the local density approximation ( lda ) , the pbe gga @xcite and the pbesol gga .
@xcite we further did full relaxations of both the @xmath10 and the @xmath12 structures using vasp with lda , pbe , and pbesol @xcite functionals .
these functionals differ in equilibrium volumes for solids .
generally , at increased volume lattices tend to distort more strongly , as was noted in the case of pbtio@xmath4 , an oxide ferroelectric with a ground state particularly sensitive to volume .
@xcite we find that the lda underestimates the unit cell volume of nasbs@xmath0 , yielding 175 @xmath13 , in comparison with the experimental volume of 192 @xmath13 at 300 k. the pbe functional yields 197 @xmath13 , while pbesol yields 184 @xmath13 .
the lda predicts a monoclinic ground state , while the fully relaxed triclinic structure is @xmath14 2 mev / atom lower in energy for both pbe and pbesol . considering the very small energy , and the limitations of dft calculations
we conclude that the ground state is monoclinic @xmath10 or possibly @xmath12 with an extremely small triclinic distortion from this monoclinic structure .
the other monoclinic structure , @xmath11 , is , however , clearly not a feasible structure .    , showing s as large red spheres , sb as gold and na as blue .
the left panel shows the layering , while the right panel shows a single layer ( note that there are two layers with opposite orientation of the s - sb - s units per cell . )
the short sb - s bonds in the s - sb - s units are shown by pipes . ]
we calculated properties for both the @xmath10 and @xmath12 structures , but find very little difference .
for example , the band gap for the triclinic structure is 1.21 ev compared to 1.22 ev for the monoclinic .
this is not surprising since the two structures are very similar , though not identical .
internal coordinates for the two structures are given in tables [ tab - mono ] and [ tab - tri ] .
as seen , the bond valences @xcite are close to their nominal values indicative of an ionic structure , na@xmath15sb@xmath16s@xmath17 , although the sb value of 2.85 is slightly smaller perhaps indicative of some degree of covalency .
.calculated atomic positions and bond valence sums for monoclinic nasbs@xmath0 , spacegroup 15 , @xmath10 , @xmath18=8.232  , @xmath19=6.836  , @xmath20=8.252  , @xmath21=55.72@xmath22 .
these lattice vectors are from experiment .
" denotes the bond - valence sum .
the fractional atomic coordinates are in terms of the lattice vectors and were determined with the pbe functional . [ cols="<,^,^,^,^",options="header " , ]     [ tab - mono - dielectric ]    therefore nasbs@xmath0 has enhanced born effective charges due to the cross gap hybridization , similar to several materials that have been found to have excellent charge collection in the context of radiation detection , e.g. tlbr , bii@xmath4 and tl@xmath23sei@xmath24 .
@xcite this enhanced born charge leads to an enhanced dielectric constant , which means enhanced screening .
this provides an explanation of how a material that presumably contains high concentrations of point defects can nonetheless have efficient carrier collection in an optoelectronic application . in this regard
, we note that srtio@xmath4 , which is near a ferroelectric transition , and consequently has a very high dielectric constant at low temperature , also has an exceptional electron mobility that exceeds 30,000 @xmath25/vs in high quality films .
we report first principles calculations for nasbs@xmath26 . we find that the ground state structure is monoclinic @xmath272/@xmath20 or possibly triclinic @xmath12 with a very small triclinic distortion .
we find highly anisotropic electronic and optical properties as may be expected based on the crystal structure .
the results show a quasidirect band gap , with an indirect gap slightly lower than the direct gap , which may impede carrier recombination .
the calculated value of the band gap using the mbj potential is 1.22 ev .
importantly , the electronic structure shows a substantial cross gap hybridization between s @xmath2 and sb @xmath2 states .
this results in enhanced born effective charges and a high dielectric constant .
this high dielectric constant provides screening and defect tolerance for the carrier collection .
therefore it is likely that nasbs@xmath0 can be a useful optoelectronic material , not only as a solar absorber , but also in applications requiring doping .
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nasbs@xmath0 is a semiconductor that was recently shown to have remarkable efficacy as a solar absorber indicating efficient charge collection even in defected material . 
 we report first principles calculations of properties that show ( 1 ) an indirect gap only slightly smaller than the direct gap , which may impede recombination of photoexcited carriers , ( 2 ) highly anisotropic electronic and optical properties reflecting a layered crystal structure , ( 3 ) a pushed up valence band maximum due to repulsion from the sb @xmath1 states and ( 4 ) cross - gap hybridization between the s @xmath2 derived valence bands and the sb @xmath3 states . 
 this latter feature leads to enhanced born effective charges that can provide local screening and therefore defect tolerance . 
 these features are discussed in relation to the performance of the compound as a semiconductor with efficient charge collection .