| The cubic double perovskite oxide crystal,A2 Ca B′O6(A=Sr,Ba;B′= Mo,W),is of great importance in research,to which many scientific workers gravitate.Although there exist a number of research articles on it,many problems remain unclear and are worth further research and exploration.In this paper,we substitute Sr for A and Mo for B′,and find their physical properties will be changed by the substitution.Here we build crystal structures for cubic double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W),in whose unit cell Sr(Ba)atom and Ca atom in 4c position are distributed symmetrically.Based on this,we compute the electronic structures,elastic and optical properties using the density functional theory(DFT)with the generalized gradient approximation(GGA),with the help of the ultra-soft plane-wave pseudo-potentials method based on the first principle density functional theory.The calculated electronic structures show that,cubic double perovskite oxide crystals A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)are all indirect band gap semiconductors,and the band gaps for Sr2 Ca Mo O6,Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 crystals are 2.02 e V,2.782 e V,2.052 e V and 2.778 e V respectively.Near the fermi surface,the top of valence band of Sr2 Ca Mo O6,Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 are chiefly donated by O-2p orbitals,while the bottom of the conduction band by strong orbital hybridization between Mo-4d(W-4d)orbital and O-2p orbital.These results suggest that d orbitals of transition metal play an important role in the electronic band structure of double perovskite crystals.The electronic cloud overlap population presents that there exists a strong covalent bond between Mo–O and W–O,whereas a weak bonding effect between Ca–O and Sr–O.Meanwhile,the covalent bond between Mo–O is stronger than that between W–O.The computational result shows that,in the Mo O6 and WO6 octahedrons of double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W),the bond length between Mo–O and W–O are shorter than the others.By comparing their crystal electronic structures,we can find that Sr2 Ca WO6 and Ba2 Ca WO6 band gap values are close to 2.78 e V,higher than the 2.0 e V of Sr2 Ca Mo O6 and Ba2 Ca Mo O6.At the same time,the density of states ofSr2 Ca WO6 and Sr2 Ca Mo O6 is close to Ba2 Ca WO6 and Ba2 Ca Mo O6 respectively,which determine their band structures and density of states mainly is B ’site element rather than little impact from A site element.We apply the stress and strain method to calculate the elastic constant(C11,C12,C44)of cubic double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)crystal.The results meet stability criteria of mechanics in cubic system.We found,after calculation of elastic property,that the B/G of Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 is smaller than 1.75,which demonstrates that these three materials are brittle in general;while the B/G of Sr2 Ca Mo O6 is larger than 2,which displays its slight ductility.The Poisson ratio of Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 is a bit smaller than that of Sr2 Ca Mo O6.That is,comparing to Sr2 Ca Mo O6,the plasticity of Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 is worse,in agreement with B/G expectation proposed by Pugh.The toughness of these four materials is ranked as the order: Ba2 Ca WO6>Ba2Ca Mo O6>Sr2Ca WO6>Sr2Ca Mo O6.The Young’s modulus E and the universal anisotropy indicator AU both suggest that all four materials are anisotropic.Therein,Sr2 Ca Mo O6 shows stronger anisotropy than Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6.The results of optical properties indicate that: the imaginary part of the dielectric function profile of cubic double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)oxide crystal resembles the same,only red shifted as the atom number of B′ climbs,which agrees with their change rule of band gap.Two double perovskite oxides with Mo,Sr2 Ca Mo O6,Ba2 Ca Mo O6,share a similar energy profile both in real part ε1(ω)and imaginary part ε2(ω)in the dielectric function.This prove that the primary factor to the dielectric function of cubic double perovskite oxide crystals A2 Ca B′O6(A=Sr,Ba;B′=Mo,W)is the element B′(B′=Mo,W).The substitution of B′ leads to the backshift of peak values in real part ε1(ω)and imaginary part ε2(ω)in the dielectric function,while the change and substitution of A(A=Sr,Ba)affect marginally its dielectric function of the optical property.Peak values of the real part ε1(ω)and imaginary part ε2(ω)of two double perovskite oxides Sr2 Ca Mo O6 and Ba2 Ca Mo O6 with Mo both are higher than two elements with W: Sr2 Ca WO6 and Ba2 Ca WO6.Four crystals own relatively strong refractivity in the energy range of the visible light.The indexes of static refraction of Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 n(0)are 2.33,2.18,2.31 and 2.17 respectively.In the range of 0 to 15 e V,the trend of cubic perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)crystals’ index of static refraction corresponds to the real part of dielectric function,and the photoconductivity to the imaginary part.In the low energy range,the absorption coefficient of Sr2 Ca WO6 is the greatest of all,exhibiting the strongest capability to absorb light.The refractivity n(ω)and reflectivity R(ω)of two double perovskite oxides Sr2 Ca Mo O6 and Ba2 Ca Mo O6 that contain Mo are larger than that contain W,Sr2 Ca WO6and Ba2 Ca WO6.For energy lower than visible light,the photoconductivity σ(ω)of double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)tend to zero.Sr2 Ca Mo O6(Ba2Ca Mo O6)and Sr2 Ca WO6(Ba2Ca WO6)reach their first peak values,2.59 fs-1(2.62 fs-1)and 2.92 fs-1(2.92 fs-1),when the energy comes to 3.82 e V(3.83 e V)and 4.59 e V(4.53 e V),where the peak value of Sr2 Ca WO6(Ba2Ca WO6)is larger than that of Sr2 Ca Mo O6(Ba2Ca Mo O6).When the energy stays at 4.82 e V,5.41 e V,4.74 e V and 5.35 e V,cubic double perovskite structures oxide crystals,Sr2 Ca Mo O6,Sr2 Ca WO6,Ba2 Ca Mo O6 and Ba2 Ca WO6 reach their first peak values: 0.25,0.19,0.27 and 0.21 respectively in energy loss function L(ω).The peak values in the energy loss match the dramatic decline in its reflection spectrums.Cubic double perovskite A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)crystals display distinct extinction and absorption property.Within the energy range of 0-15 e V,cubic double perovskite oxide crystals A2 Ca B′O6(A=Sr,Ba;B′= Mo,W)retain a comparatively short energy loss range,where their energy loss restricts to certain energy range.So they can be preferable material for optical storage. |
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