Pressure-induced structural transition in chalcopyrite ZnSiP₂

II-IV-V₂ chalcopyrites represent a promising class of materials for Si-based tandem solar cells.^1–3 These ternary compounds are analogous to III–V materials wherein the group III element is replaced with a group II and a group IV element, and the structure type changes from sphalerite to chalcopyrite. Due to their physical and chemical properties, these compounds are found to be suitable candidates for many technological applications in the areas of infrared emission and detection and optoelectronics.^1–9 ZnSiP₂ is particularly intriguing due to its close lattice matching with Si, direct bandgap near 2.0 eV, and photovoltaic response.^1,2

Studies of semiconducting compounds under pressure provide an opportunity to examine structure-property relationships and pressure-induced phase transitions, in particular. Pressure is an external perturbative variable for tuning materials' properties whereby the redistribution of electron densities and changes in the nature of chemical bonds may occur.¹⁰ Previous studies of chalcopyrite semiconductor^11–16 compounds under pressure indicate a general tendency of transition from tetrahedral coordination at low pressure to denser phases with octahedral coordination. Most of these compounds undergo a phase transition to the rock salt-type structure, which is characterized by the onset of disorder with cations distributed statistically among the octahedral sites. In the case of CuGaS₂ and AgGaS₂, the transition to rock salt occurs at 16 and 12 GPa, respectively.^11,12 Some compounds initially transition to the α-NaFeO₂ type structure.¹¹ Raman spectroscopy is an appropriate tool to probe this phase transition as spectral features were found to undergo substantial changes around the phase transition pressure.^12–14 In most cases, the chalcopyrite to rock salt transition is irreversible, and the recovered material is either amorphous or exhibits a crystal structure different from that of the high-pressure disordered phase. Thus, investigation of the pressure-induced phase behavior of chalcopyrite-type semiconductors is of interest for establishing the possibility of new II-IV-V₂ polytypes with different optical and electronic properties. For example, the optical bandgap of ZnSnP₂ can be tuned by stabilizing it in either the chalcopyrite- or sphalerite-type structure by varying the crystal growth conditions.¹⁷ 

Synthesis and characterization of bulk ZnSiP₂ single crystals are well documented.^18–21 There have been efforts to grow ZnSiP₂ thin films on Si using various thin film growth techniques,^22,23 yet high-pressure studies on ZnSiP₂ are limited.^24–28 Often times, interfacial stress caused by lattice mismatch and structural defects in thin films may lead to stabilization of metastable phases.^29,30 Thus, the knowledge of external stress effects on the phase behavior of bulk ZnSiP₂ would be helpful in various aspects of thin film growth and characterization. Previous theoretical studies predicted that ZnSiP₂ would undergo a phase transition to the rock salt structure between 24 and 35 GPa.^26,27 However, the experimental studies on this system so far are confined to the 0–5 GPa range.^24,25 Here, we report the structure and phonon evolution of ZnSiP₂ up to 30 GPa and explore the phase behavior of this material under pressure. In situ synchrotron x-ray diffraction (XRD) and Raman scattering inside a diamond anvil cell (DAC) were used to probe the crystal structure and phonons, respectively. The complete experimental details are included in the supplementary material.

X-ray diffraction (XRD) patterns of ZnSiP₂ as a function of pressure are shown in Fig. 1. The pattern at ambient pressure can be easily indexed and refined to the expected tetragonal I$4¯$2d structure with lattice parameters (a = 5.3929(8) and c = 10.4320(16)), in close agreement with earlier reports and as-synthesized crystals.^20,21 Apart from the diffraction from ZnSiP₂, there are additional Bragg lines which originate from NaCl and/or He/Ar that served as additional pressure markers and the pressure transmitting media (PTM). The structure of chalcopyrite ZnSiP₂ can be derived from sphalerite (zinc-blende) by subdividing the fcc cation lattices into two symmetric parts such that each anion has nearest neighbors of each cation type (as shown in Fig. 1). The volume of the tetragonal unit cell of the chalcopyrite structure is twice that of the zinc-blende unit cell and consists of four formula units per unit cell. The Zn atoms occupy the 4a (0, 0, 0) sites, Si atoms occupy the 4b (0, 0, 0.5) sites, and P atoms occupy the 8d (u, 0.25, 0.125) sites. Here, u is the structure parameter that represents the extent of tetrahedral distortion. The distortion is due to the displacement of P atoms from the center of the tetrahedron towards one of the cations, leading to two distinct Zn–P and Si–P bond lengths. The parameter u can be represented as a function of those bond lengths and the lattice parameter a.^27,31 The pressure-volume (P-V) data of the chalcopyrite phase collected during compression runs were fitted with a 2nd order Birch-Murnaghan equation of state (EoS) (see the inset of Fig. 1), and the estimated zero-pressure bulk modulus (K₀) is 83(4) GPa, which is comparable to that of analogous III–V materials like GaP.³² 

The Bragg peaks from the sample start broadening with increasing pressure, but the structure remains as tetragonal chalcopyrite to 25 GPa. The broadening of Raman lines under pressure is not only typically caused by non-hydrostatic stresses associated with the PTM but could also be associated with other effects such as lattice disorder. Since we used He as the PTM, we anticipate that non-hydrostaticity and its effect on the line broadening are minimal.³³ Hence, the line broadening could be an indication of the onset of lattice disorder.

Above 25 GPa, the sample undergoes a structural phase transition and diffraction from the sample consisting of mostly broad features, while some sharp lines remain in NaCl and the pressure transmitting medium. The peaks from NaCl and helium can be easily indexed, given that their phase diagrams are well established. The broad Bragg peaks originating from the sample could be indexed to a rock salt–type (space group-Fm$3¯$m) structure with the lattice parameter, a ≈ 4.68 Å. Based on the structural model reported in the literature for the rock salt phases of similar materials,^11,14,34 it is found that Zn/Si atoms occupy (0, 0, 0) and P atoms occupy (0.5, 0.5, 0.5) positions with a total of two formula units per unit cell. As shown in Fig. 1, this model explains well the observed X-ray diffraction pattern of ZnSiP₂ in the disordered rock salt phase. The lattice volume change per atom between the chalcopyrite structure at 25.8 GPa and the disordered rock salt-type structure at 30 GPa is approximately 17%. It is to be noted that the structural model for the rock salt phase suggested in some theoretical reports^15,16,26 does not match with our observations. Previous calculations used ordered rock salt-type structural models that result in significantly larger lattice parameters²⁶ than what we observe here experimentally. In addition to the Bragg lines from the rock salt-type structure, there are additional weak peaks (denoted with asterisks (*) in Fig. 1) that are unaccounted for. These peaks are consistent with a weak orthorhombic distortion (as is observed for other related materials at high pressure), but it was not possible to accurately quantify the degree of such a distortion from our data. Nevertheless, the disordered rock salt-type nature of the high-pressure phase is consistent with previously reported pressure-induced, order-disorder phase transitions in chalcopyrite materials.^11,12

As per the model suggested by Zunger³¹ for the temperature-dependent phase behavior of ternary ABC₂ chalcopyrite systems, the difference in A–C and B–C bond lengths (i.e., tetrahedral distortion) increases with temperature for the ordered chalcopyrite phase. In the disordered phase, there will be distribution of bond lengths reflecting the different possible local environments that the C atom may have around it due to cationic disorder. This mechanism could also be applied to the case of pressure, given that the nature of the order-disorder phase transition is the same, as has already been observed in some ternary chalcopyrite systems. In most of the cases, the high-pressure phase exhibits a crystal structure wherein the anion is octahedrally coordinated by the cations. Compared to other chalcopyrite systems, the transition pressure of 25–30 GPa in ZnSiP₂ is significantly higher. As per earlier reports, the transition pressure in tetrahedrally bonded ABX₂ compounds could be related to the ionicity of the A–X bond.^27,35 A clear trend between the increase in Cu–X bond ionicity and the corresponding increase in transition pressure could be seen in CuBX₂ (B: Al, Ga, and In; X: S, Se, and Te) compounds.^11–14,36 Based on this, it is plausible to say that the high transition pressure in ZnSiP₂ may reflect the large ionicity of the Zn–P bond.^26,37

Upon slow (hours) decompression, the disordered high-pressure phase remains stable down to 11 GPa, below which it undergoes a phase transition to another disordered phase that is recoverable to ambient conditions (see Fig. S1 in the supplementary material). To avoid the interference of peaks from the pressure standard in the XRD pattern, we have repeated the experiment without NaCl, and the diffraction pattern of the recovered sample is shown in Fig. 2. It consists of broad features in combination with relatively sharp and well-defined peaks. The relative intensities of the broad and sharp peaks vary depending on the measurement position, indicating the coexistence of both amorphous and crystalline phases. After subtraction of the amorphous contribution (see Fig. 2), the relatively sharp peaks can be easily indexed to the chalcopyrite-type structure with lattice parameters that are ∼1.2% larger than that of the starting material. It is apparent that during decompression, the material “tries” to come back to its original chalcopyrite phase, but due to the presence of cation disorder, the material is partially amorphized.

In order to check for possible chemical disproportionation in the recovered sample, energy dispersive X-ray spectroscopy (EDS) measurements were performed. As shown in the inset of Fig. 2, the EDS layered-image indicates chemical homogeneity across the entire sample area. This observation indicates that there is no measureable extent of chemical disproportionation within the recovered sample at the length scales considered and that both the amorphous/disordered and crystalline regions of the sample maintain the ZnSiP₂ composition.

Optical absorbance measurements indicate a redshift in the absorption edge of ZnSiP₂ with increasing pressure before the onset of the phase transition (see Fig. S2 in the supplementary material). The sample turns dark, and all transmission (in the visible and near-infrared (NIR) range) is lost after the phase transition, suggesting the pressure-induced metallization of the system. No measureable optical transmission was observed in the recovered sample, possibly due to the coexistence of both semiconducting (chalcopyrite) and absorbing (amorphous) phases.

As per the group symmetry rules, there are 13 Raman active modes in ZnSiP₂.²¹ The first-order Raman spectrum of ZnSiP₂ at room temperature as a function of pressure is shown in Fig. 3(a). The mode assignment was performed based on the previous reports.^21,24,38 The mode frequencies below 200 cm⁻¹ correspond to acoustic zone-center phonon modes, and the modes between 200 cm⁻¹ and 550 cm⁻¹ are optical phonon modes. The pressure-dependent behavior of the Raman modes below 5 GPa agrees well with the previous report.²⁴ The low-frequency acoustic mode is a soft mode, and its frequency decreases monotonically until 27 GPa; above this pressure, the mode is absent. The soft-mode behavior indicates the mechanical instability and tendency towards a phase transition. This soft-mode behavior has been observed in other chalcopyrite semiconductors when they undergo phase transitions to a high symmetry (usually NaCl-type structure) phase at high pressures.^12,39–41 The color change of the sample (see the inset of Fig. 3(a)) along with the disappearance of optical transmission (see Fig. S2 in the supplementary material) and Raman signal (see Fig. 3(a)) after the phase change all indicate that the high-pressure phase is probably metallic.

The pressure dependence of the phonon frequencies estimated from the data collected using He as the PTM is plotted in Fig. 3(b). The solid lines in the figure represent a polynomial fit of the form

where ω and ω₀ are phonon frequencies (in cm⁻¹) and P is the pressure (in GPa). Using this P-ω relation and the bulk modulus estimated from the P-V data, we calculated the mode Grüneisen parameter (γ) for each mode. The parameters ω₀, a, b, c, and γ are summarized in Table I. As expected, the γ value is negative for the soft mode due to its monotonic decrease in the frequency with pressure. The negative Grüneisen parameter is an indication of lattice instability along with the disorder causing the phase transition. Due to lattice instability, the tetragonal distortion vanishes and the system undergoes an octahedral distortion in the high-pressure phase.

The estimated γ values are significantly different from the ones reported before.²⁴ The reason is threefold: (1) in the previous report, the bulk modulus of ZnSiP₂ was assumed to be equivalent to that of GaP; (2) previous experiments were limited to 5 GPa within which the P-ω relationship is mostly linear; and (3) non-hydrostaticity of the PTM used in the previous experiments. Since we used He as the PTM, we would expect a more hydrostatic P-ω relationship in our measurements, which would facilitate more accurate γ values.

With the exception of the low-frequency soft mode, the other modes initially harden as a function of pressure. However, the low-frequency modes (B₁ and E) between 130 and 200 cm⁻¹ initially harden only until 5–15 GPa and soften beyond that pressure (see Fig. 4). To further understand this behavior, we have plotted the full-width-at-half-maxima (FWHM) of these modes as a function of pressure in Fig. 4. It is evident that the FWHM also undergo anomalous changes between 5 and 15 GPa. Correspondingly, normalized intensities of these modes also undergo anomalous changes (see Fig. S3 in the supplementary material). It is to be noted that there is no structural change observed in this pressure range. Analogous behavior was reported previously in tetragonally bonded AB₂C₄ semiconductors that crystallize in the defect chalcopyrite structure.^38,40 The pressure-induced, order-disorder phase transition in these materials occurs in two stages. The first stage of the order-disorder phase transition is associated with the softening of the low-frequency modes and observable changes in their FWHM and intensities, which are attributed to partial disorder in the cation sublattice.³⁹ The second stage involves the complete disappearance of the Raman modes, which is attributed to the structural transition to the rock salt-type structure. Although there are no reports on similar two-stage, order-disorder phase transitions in chalcopyrite ABC₂ systems, based on the similarity of the behavior of the Raman modes, it is plausible to attribute the changes in the low-frequency Raman modes between 5 and 15 GPa as a prelude to the high-pressure, order-disorder phase transition.

In summary, we have studied the phase behavior of chalcopyrite ZnSiP₂ under pressure up to 30 GPa. Consistent with prior theoretical predictions, ZnSiP₂ undergoes an order-disorder structural phase transition between 27 and 30 GPa. The disordered nature of the high-pressure phase was reflected by the appearance of broad peaks in the X-ray diffraction pattern, which were indexed to a rock salt type structure or a slightly distorted orthorhombic variant. The phase transition is also marked by the soft-mode behavior of low-frequency acoustic phonons and the disappearance of the Raman signal and optical transmission. In addition, the low-frequency Raman mode parameters (FWHM and intensities) undergo anomalous changes between 5 and 15 GPa, suggesting a two-stage, order-disorder phase transition mechanism for this material. Upon slow decompression, the recovered sample exhibits the chalcopyrite-type structure with partial amorphization and uniform chemical composition.

See supplementary material for the experimental methods, optical absorbance, and additional Raman data analysis.

We thank M. Ward and H. Gou for assistance with XRD measurements. This work was supported by Energy Frontier Research in Extreme Environments (EFree) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science under award No. DE-SC0001057. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. RH acknowledges the support of DOE-BES/DMSE under Award DE-FG02-99ER45775. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, sector 13). GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy - GeoSciences (DE-FG02-94ER14466). Partial support for the ZnSiP₂ crystal growth was provided by the National Science Foundation under grant No. 1555340.