Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn₂Bi₂

Abstract

In-situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn2Bi2 undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. CaMn2Bi2 displays a new structure type above 2.3 GPa, with the puckered Mn honeycomb lattice of the trigonal ambient-pressure structure converting to one-dimensional (1D) zigzag chains in the high-pressure monoclinic structure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic two order of magnitude drop of resistivity; although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as, surprisingly, are two resistivity anomalies with opposite pressure dependences. Based on the electronic structure calculations, we hypothesized that the newly emerged electronic state under high pressure is associated with a Fermi surface instability of the quasi-1D Mn chains, while we infer that the other is a magnetic transition. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn2Bi2 indicates that ferrimagnetism is thermodynamically favored.

Anisotropic Structural Collapse of Mg3Sb2 and Mg3Bi2 at High Pressure

“…The zero-pressure bulk modulus of AP- and HP-Mg 3 Sb 2 is 38 and 46 GPa, respectively, while the zero-pressure bulk modulus of AP- and HP-Mg 3 Bi 2 is 37 GPa and 49 GPa, respectively. The HP structures of both compounds are slightly stiffer than the AP structures, similar to the behavior reported for CaMn 2 Bi 2 . The zero-pressure bulk moduli of AP-Mg 3 Sb 2 and AP-Mg 3 Bi 2 obtained in this study are comparable to the results of resonant ultrasound spectroscopy (36 and 38 GPa, respectively) and DFT (42 and 37 GPa, respectively) …”

“…The HP structures of both compounds are slightly stiffer than the AP structures, similar to the behavior reported for CaMn 2 Bi 2 . 21 The zero-pressure bulk moduli of AP-Mg 3 Sb 2 and AP-Mg 3 Bi 2 obtained in this study are comparable to the results of resonant ultrasound spectroscopy (36 and 38 GPa, respectively) 14 and DFT (42 and 37 GPa, respectively). 37 The question of whether or not Mg 3 Pn 2 (Pn = Sb, Bi) are layered structures has been under debate.…”

“…The tetrahedrally coordinated (Mg2) 2 Sb 2 slab in AP-Mg 3 Sb 2 (shown in blue/cyan) transforms at high pressure into a layer with alternating tetrahedral and square pyramidal coordination environments. In this respect, HP-Mg 3 Sb 2 has structural similarities to previously reported HP-CaMn 2 Bi 2 (space group P 2 1 / m ), which exhibits a very similar Mn 2 Bi 2 slab at pressures above 2 GPa. However, HP-Mg 3 Sb 2 shows key differences with HP-CaMn 2 Bi 2 with respect to the octahedral Mg1 layer shown in red; in HP-Mg 3 Sb 2 , every other octahedron distorts to form a square-planar environment, accompanied by the breaking of two opposing Mg(1)–Sb bonds.…”

“…Despite intense interest in these compounds in recent years, no experimental high-pressure investigation exists to date. In fact, there have been only a handful of high-pressure studies of compounds in the CaAl 2 Si 2 structure type, − despite their long history and potential technological importance. Here, we report on the discovery of a previously unrecognized high-pressure phase transition in Mg 3 Pn 2 (Pn = Sb, Bi) using in situ high-pressure synchrotron X-ray diffraction (XRD) combined with first-principles calculations.…”

Competition between electronic correlations and hybridization in CaMn$_{2}$Bi$_{2}$

“…Recent experimental and theoretical studies find CaMn 2 Bi 2 to host many intriguing properties including large anisotropic magnetoresistance 16 and a planeto-chain structural transition 17 , but most curiously it has been suggested that CaMn 2 Bi 2 may be a hybridizationgap semiconductor. 18,19 In line with this claim, lowtemperature electrical transport measurements find a slight increase of the gap under pressure.…”