arXiv subjects

Alexandra Zevalkink

Publications and source records attributed to Alexandra Zevalkink.

3 recordsLinked to original sources

arXiv2026

Compressibility and High-Pressure Structure of CaMg$_2$Bi$_2$ and YbMg$_2$Bi$_2$

Compounds with the formula $AM_2X_2$ in the CaAl$_2$Si$_2$ structure type have garnered increasing interest across various solid-state research domains, such as quantum topological and thermoelectric materials. Prior studies have identified high-pressure phase transitions in several compounds, including Mg$_3$Sb$_2$, Mg$_3$Bi$_2$, CaMn$_2$Bi$_2$, and SrAl$_2$Si$_2$. In this study, we investigate the structural behavior of CaMg$_2$Bi$_2$ and YbMg$_2$Bi$_2$ under varying pressure conditions. We synthesized crystals using the molten metal flux method and examined them through single-crystal synchrotron X-ray diffraction, employing diamond anvil cells to exert pressures up to 20 GPa. Our analysis reveals insights into the anisotropic compressibility of these materials, highlighting the more compressible and flexible octahedral $A$-Bi bonds as the primary contributors to this anisotropy. Moreover, we observed a phase transition in both CaMg$_2$Bi$_2$ and YbMg$_2$Bi$_2$ at pressures above 9.6 GPa and 8.7 GPa, respectively. The newly identified high-pressure phase exhibits a distortion of the original CaAl$_2$Si$_2$ structure with space group $C2/m$. This high-pressure structure is distinct from that of related compounds (e.g., CaMn$_2$Bi$_2$, MgMg$_2$Bi$_2$), the latter exhibiting a square pyramidal coordination for the $M$ site.

cond-mat.mtrl-sci↗

arXiv2024

Bonding Interactions Can Drive Topological Phase Transitions in a Zintl Antiferromagnetic Insulator

While $\sim$30% of materials are reported to be topological, topological insulators are rare. Magnetic topological insulators (MTI) are even harder to find. Identifying crystallographic features that can host the coexistence of a topological insulating phase with magnetic order is vital for finding intrinsic MTI materials. Thus far, most materials that are investigated for the determination of an MTI are some combination of known topological insulators with a magnetic ion such as MnBi$_2$Te$_4$. Motivated by the recent success of EuIn$_{2}$As$_{2}$, we investigate the role of chemical pressure on topologically trivial insulator, Eu$_5$In$_2$Sb$_6$ via Ga substitution. Eu$_5$Ga$_2$Sb$_6$ is predicted to be topological but is synthetically difficult to stabilize. We look into the intermediate compositions between Eu$_5$In$_2$Sb$_6$ and Eu$_5$Ga$_2$Sb$_6$ through theoretical works to explore a topological phase transition and band inversion mechanism. We attribute the band inversion mechanism to changes in Eu-Sb hybridization as Ga is substituted for In due to chemical pressure. We also synthesize Eu$_{5}$In$_{4/3}$Ga$_{2/3}$Sb$_{6}$, the highest Ga concentration in Eu$_{5}$In$_{2-x}$Ga$_{x}$Sb$_{6}$, and report the thermodynamic, magnetic, transport, and Hall properties. Overall, our work paints a picture of a possible MTI via band engineering and explains why Eu-based Zintl compounds are suitable for the co-existence of magnetism and topology.

cond-mat.mtrl-sci↗

arXiv2018

An unlikely route to low lattice thermal conductivity: small atoms in a simple layered structure

In the design of materials with low lattice thermal conductivity, compounds with high density, low speed of sound, and complexity at either the atomic, nano- or microstructural level are preferred. The layered compound Mg$_3$Sb$_2$ defies these prevailing paradigms, exhibiting lattice thermal conductivity comparable to PbTe and Bi$_2$Te$_3$, despite its low density and simple structure. The excellent thermoelectric performance ($zT$ $\sim$ 1.5) in $n$-type Mg$_3$Sb$_2$ has thus far been attributed to its multi-valley conduction band, while its anomalous thermal properties have been largely overlooked. To explain the origin of the low lattice thermal conductivity of Mg$_3$Sb$_2$, we have used both experimental methods and ab initio phonon calculations to investigate trends in the elasticity, thermal expansion and anharmonicity of $A$Mg$_2Pn_2$ Zintl compounds with $A$ = Mg, Ca, Yb, and $Pn$ = Sb and Bi. Phonon calculations within the quasi-harmonic approximation reveal large mode Grüneisen parameters in Mg$_3$Sb$_2$ compared with isostructural compounds, in particular in transverse acoustic modes involving shearing of adjacent anionic layers. Measurements of the elastic moduli and sound velocity as a function of temperature using resonant ultrasound spectroscopy provide a window into the softening of the acoustic branches at high temperature, confirming their exceptionally high anharmonicity. We attribute the anomalous thermal behavior of Mg$_3$Sb$_2$ to the diminutive size of Mg, which may be too small for the octahedrally-coordinated site, leading to weak, unstable interlayer Mg-Sb bonding. This suggests more broadly that soft shear modes resulting from undersized cations provide a potential route to achieving low lattice thermal conductivity low-density, earth-abundant materials.

cond-mat.mtrl-sci↗