Defect Stannite Research Articles

Optical absorption of defect chalcopyrite and defect stannite ZnGa2Se4 under high pressure

Optical absorption measurements at high pressure have been performed in two phases of the ordered-vacancy compound (OVC) ZnGa2Se4: defect stannite (DS) and defect chalcopyrite (DC). The direct bandgap energy of both phases exhibits a non-linear pressure dependence with a blueshift up to 10 GPa and a redshift at higher pressures. We discuss the different behavior of both phases in these two pressure ranges in relation to the pressure-induced order-disorder processes taking place at cation sites. Measurements performed in both phases on downstroke after increasing pressure to 22 GPa show that the direct bandgap energy of the recovered samples at room pressure was 0.35 eV smaller than that of the original samples. These results evidence that different disordered phases are formed on decreasing pressure, depending on the cation disorder already present in the original samples. In particular, we attribute the recovered samples from the original DC and DS phases to disordered CuAu (DCA) and disordered zincblende (DZ) phases, respectively. The decrease of the direct bandgap energy and its pressure coefficient on increasing disorder in the four measured phases are explained. In summary, this combined experimental and theoretical work on two phases (DC and DS) of the same compound has allowed us to show that the optical properties of both phases show a similar behavior under compression because irreversible pressure-induced order-disorder processes occur in all adamantine OVCs irrespective of the initial crystalline structure.

Order/disorder phenomena in Zn1−xMnxGa2Se4 ordered vacancy compounds: high temperature neutron powder diffraction experiments

We present a study of order–disorder phenomena in the series of tetrahedral ordered vacancy compounds Zn1−xMnxGa2Se4 by means of time-of-flight neutron diffraction at high temperature together with dc magnetic susceptibility, Raman spectroscopy, differential thermal analysis and optical absorption experiments. Samples of nominal composition x = 0, 0.24, 0.5, 0.77 and 1 have been studied. An order–disorder phase transition has been detected, with Tc ranging from 472 to 610 ° C, which involves a structural change from a defect chalcopyrite phase, with space group (s.g.) and three different cation sites, to a partially disordered defect stannite, in which Zn, Mn and half of the Ga ions share the 4d site in s.g. Neither the vacancies nor the Ga ions occupying site 2a are involved in the phase transition. An additional ordering process is observed on approaching the phase transition from below, which is attributed to several factors: the activation of cation diffusion at ∼300 ° C, the partially disordered cation distribution exhibited by the as-grown single crystals and the preference of Mn atoms for the 2d crystallographic site in the structure. The reversibility of the phase transition is analysed with the aid of magnetic, optical and Raman experiments.

High-pressure Raman scattering study of defect chalcopyrite and defect stannite ZnGa2Se4

High-pressure Raman scattering measurements have been carried out in ZnGa2Se4 for both tetragonal defect chalcopyrite and defect stannite structures. Experimental results have been compared with theoretical lattice dynamics ab initio calculations and confirm that both phases exhibit different Raman-active phonons with slightly different pressure dependence. A pressure-induced phase transition to a Raman-inactive phase occurs for both phases; however, the sample with defect chalcopyrite structure requires slightly higher pressures than the sample with defect stannite structure to fully transform into the Raman-inactive phase. On downstroke, the Raman-inactive phase transforms into a phase that could be attributed to a disordered zincblende structure for both original phases; however, the sample with original defect chalcopyrite structure compressed just above 20 GPa, where the transformation to the Raman-inactive phase is not completed, returns on downstroke mainly to its original structure but shows a new peak that does not correspond to the defect chalcopyrite phase. The pressure dependence of the Raman spectra with this new peak and those of the disordered zincblende phase is also reported and discussed.

Thermally activated cation ordering in ZnGa2Se4 single crystals studied by Raman scattering, optical absorption, and ab initio calculations

Order–disorder phase transitions induced by thermal annealing have been studied in the ordered-vacancy compound ZnGa2Se4 by means of Raman scattering and optical absorption measurements. The partially disordered as-grown sample with tetragonal defect stannite (DS) structure and space group has been subjected to controlled heating and cooling cycles. In situ Raman scattering measurements carried out during the whole annealing cycle show that annealing the sample to 400 °C results in a cation ordering in the sample, leading to the crystallization of the ordered tetragonal defect chalcopyrite (DC) structure with space group. On decreasing temperature the ordered cation scheme of the DC phase can be retained at ambient conditions. The symmetry of the Raman-active modes in both DS and DC phases is discussed and the similarities and differences between the Raman spectra of the two phases emphasized. The ordered structure of annealed samples is confirmed by optical absorption measurements and ab initio calculations, that show that the direct bandgap of DC-ZnGa2Se4 is larger than that of DS-ZnGa2Se4.

Order–disorder processes in adamantine ternary ordered‐vacancy compounds

AbstractAdamantine ternary ordered‐vacancy compounds (OVCs) of the AB2X4 family derive from the zincblende structure of binary AX compounds and share many properties with the chalcopyrite‐type ternary ABX2 compounds. AB2X4 mainly crystallize in the defect chalcopyrite (or thiogallate) structure but also in other vacancy‐ordered phases, like the defect stannite (DS, or defect famatinite) and the pseudocubic (PC) structures, where vacancies occupy a fixed Wyckoff site in an ordered and stoichiometric fashion. Order–disorder phase transitions have been studied in several adamantine OVCs at high temperatures and/or pressures, where OVCs undergo a phase transition to a disordered zincblende (DZ) structure on increasing temperature and to a disordered rocksalt (DR) structure on increasing pressure. It has been suggested that these order–disorder transitions should undergo via intermediate phases of partial disorder between the fully ordered phases and the fully disordered phases. In this work we study the different possible intermediate phases of partial disorder and discuss the possibility to find them with the help of vibrational spectroscopies, like Raman scattering or infrared measurements.

Vibrational study of HgGa2S4 under high pressure

In this work, we report on high-pressure Raman scattering measurements in mercury digallium sulfide (HgGa2S4) with defect chalcopyrite structure that have been complemented with lattice dynamics ab initio calculations. Our measurements evidence that this semiconductor exhibits a pressure-induced phase transition from the completely ordered defect chalcopyrite structure to a partially disordered defect stannite structure above 18 GPa which is prior to the transition to the completely disordered rocksalt phase above 23 GPa. Furthermore, a completely disordered zincblende phase is observed below 5 GPa after decreasing pressure from 25 GPa. The disordered zincblende phase undergoes a reversible pressure-induced phase transition to the disordered rocksalt phase above 18 GPa. The sequence of phase transitions here reported for HgGa2S4 evidence the existence of an intermediate phase with partial cation-vacancy disorder between the ordered defect chalcopyrite and the disordered rocksalt phases and the irreversibility of the pressure-induced order-disorder processes occurring in ordered-vacancy compounds. The pressure dependence of the Raman modes of all phases, except the Raman-inactive disordered rocksalt phase, have been measured and discussed.

Optical phonon modes and structure of ZnGa 2Se 4 and ZnGa 2S 4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa 2Se 4 and ZnGa 2S 4. Most of the A II B 2 III C 4 VI compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group I 4 ¯ where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry I 4 ¯ 2 m (defect stannite), has been reported. For ZnGa 2Se 4 we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group I 4 ¯ for the investigated crystals of ZnGa 2Se 4. Regarding ZnGa 2S 4 we have found three modes exclusively showing Raman activity (2 A⊕1 B 1), and only eight modes showing infrared as well as Raman activity (3 B 2⊕5 E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of A II B 2 III C 4 VI compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa 2S 4 most likely crystallizes in space group I 4 ¯ 2 m .

Bond properties of the chalcopyrite and stannite phases in the Cu–(In,Ga)–Se system

Bond properties of the chalcopyrite and (defect) stannite phases in the Cu–(In,Ga)–Se system are compared in view of the bond overlap population calculated by the molecular orbital calculation of the DV-Xα method. Bond stretching force constant α is estimated for the stannite phases through the bond Ovlp. The Cu–Se and In(Ga)–Se bonds in defect stannite structure are considered to be mechanically weakened by the 2b-site vacancies. We estimate the weakened force constants to be 60–70% of those in the chalcopyrite structure. On the other hand, in In(Ga)-rich stannite, In(Ga) 4d–Se 8i and In(Ga) 2b–Se 8i bonds are estimated to be tighter by 23–25 and 8–9%, respectively, than In(Ga) 4b–Se 8d bond in the chalcopyrite structure. The Γ 1 frequencies of the stannite phases are also calculated using the estimated force constants. Characteristic Raman signals peaked at 160–175 cm −1 observed for the Cu(In 1− x Ga x ) 3Se 5 system are explained by the Cu-rich phase for the Cu–In–Se system, and the phase combination of Cu-rich and Ga 2aV 2b types for the Cu–Ga–Se system from these calculations.

Lattice dynamics of the chalcopyrite and defect stannite phases in the Cu–(In, Ga)–Se system

Raman spectra observed for bulk crystals of defect stannite phases in the Cu–(In, Ga)–Se system are analyzed mainly by the dispersion curve calculations with a superposed lattice model, assuming four fundamental structures. The calculations are performed on the basis of the Keating model, solving dynamical matrix of 24×24. Vacancy induced modifications on force interactions between nearest atoms are also discussed with effective bond overlap population and effective charges determined by the DV-Xα calculations for unit cell clusters. Apparently, invariable Raman profiles with respect to the In to Ga molar ratios through the alloy systems are explained by the superposed lattice model.

Vacancy ordering/disordering and electronic structures of II 1III 2VI 4 compounds

Electronic structures of II 1III 2VI 4 compounds are calculated by the first-principles method, for four different vacancy-ordering structures; defect chalcopyrite, defect stannite, pseudocubic and its family. It is shown that the valence-band top is made of dangling bonds of anion atoms around vacancies and its energy position hardly depends on the vacancy ordering. On the other hand, since the lowest conduction band is made the anti-bonding states of cation atoms and the cation-site vacancy acts as a potential barrier for electrons, the band-gap energy remarkably depends on the vacancy ordering.

Raman Spectra of Defect Stannite Phases in the Cu-(In,Ga)-Se System

Lattice vibration behaviors of defect stannite phases in the Cu-(In,Ga)-Se system are studied by the Raman spectrometry and the dispersion curve calculation in comparison with the chalcopyrite phases in this system. Bulk crystals in the alloy systems of Cu(In1-xGax)3Se5 and Cu2(In1-xGax)4Se7 are prepared by the normal freezing method. The characteristic two Raman peaks at 160 and 175 cm-1 for these crystals, apparently unrelated to the indium to gallium ratio, are assigned as two Γ1 modes of D2d symmetry. This can be explained from the dispersion curve calculation assuming the vacancies at 2a or 2b site of the defect stannite structure through the Cu-(In,Ga)-Se system.

Fundamental Optical Absorption Edge in MnIn2Te4 Single Crystals

The optical properties are studied in the region of the fundamental absorption edge of MnIn2Te4 single crystals (142m, defect stannite) oriented along (100) and (001). The width of the energy gap is measured to be Eg = 1.55, 1.425, and 1.41 eV, for the temperatures 77, 300, and 370 K, respectively. The α(ℏω) spectra are established to follow Urbach's rule throughout the temperature range studied, the steepness of the absorption edge remaining constant. Crystal annealing followed by fast cooling shifts the absorption edge longward by 50 meV, the exponential character of α(ℏω) persisting within the whole range T = 77 to 370 K. An analysis shows the optical absorption in the region of the fundamental edge to be governed by the sum of the effects of the density-of-states tails associated with a high concentration of lattice vacancies and of the electron-phonon interaction. The optical linear dichroism of the absorption edge of MnIn2Te4 crystals with defect stannite structure is discovered and studied. It is found that the crystals belonging to the tetragonal syngony, such as MnIn2Te4 (142m) and CuInSe2, CuInTe2 (142d), and exhibiting a weak tetragonal lattice deformation τ ≈ 0, reveal, in contrast to most chalcopyrites with τ > 0, a positive linear dichroism of transmittance close to Eg. Minimal direct transitions in MnIn2Te4 are allowed in the E ⊥ c polarization, the crystal splitting of the valence band at the Γ point being positive. [Russian Text Ignored].