Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 1901.00183

Spontaneous non-stoichiometry and ordering of metal vacancies in degenerate insulators

Oleksandr I. Malyi·Michael T. Yeung·Kenneth R. Poeppelmeier·Clas Persson·Alex Zunger
View original source ↗Read source document ↗Export metadata ↓

Abstract

We point to a class of materials representing an exception to the Daltonian view that compounds maintain integer stoichiometry at low temperatures, because forming stoichiometry-violating defects cost energy. We show that carriers in the conduction band (CB) of degenerate insulators in Ca-Al-O, Ag-Al-O, or Ba-Nb-O systems can cause a self-regulating instability, whereby cation vacancies form exothermically because a fraction of the free carriers in the CB decay into the hole states formed by such vacancies, and this negative electron-hole recombination energy offsets the positive energy associated with vacancy bond breaking. This Fermi level-induced spontaneous non-stoichiometry can lead to the formation of crystallographically ordered vacancy compounds (OVCs), explaining the previously peculiar occurrence of unusual atomic sequences such as BalNbmOn with l:m:n ratios of 1:2:6, 3:5:15, or 9:10:30. This work has general ramifications as the degenerate insulators have found growing interests in many fields ranging from transparent conductors to electrides that are electron donating promotors for catalyst.

Explore related subjects

cond-mat.mtrl-sci

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Oleksandr I. Malyi, Michael T. Yeung, Kenneth R. Poeppelmeier, Clas Persson, Alex Zunger. 2019-01-01. Spontaneous non-stoichiometry and ordering of metal vacancies in degenerate insulators. https://doi.org/10.1016/j.matt.2019.05.014

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Structure and dynamics of the negative thermal expansion material Cd(CN)$_2$ under hydrostatic pressure

We use a combination of variable-temperature / variable-pressure neutron powder diffraction, variable-pressure inelastic neutron scattering, and quantum chemical calculations to interrogate the behaviour of the negative thermal expansion (NTE) material $^{114}$Cd(CN)$_2$ under hydrostatic pressure. We determine the equation of state of the ambient-pressure phase, and discover the so-called `warm hardening' effect whereby the material becomes elastically stiffer as it is heated. We also identify a number of high-pressure phases, and map out the phase behaviour of Cd(CN)$_2$ over the range $0\leq p\leq0.5$\,GPa, $100\leq T\leq300$\,K. As expected for an NTE material, the low-energy phonon frequencies are found to soften under pressure, and we determine an effective Gr{ü}neisen parameter for these modes. Finally, we show that the elastic behaviour of Cd(CN)$_2$ is sensitive to the local Cd coordination environment, which suggests an interplay between short- (phononic) and long-timescale (cyanide flips) fluctuations in Cd(CN)$_2$.

cond-mat.mtrl-sci↗
arXiv2026

Unconventional Magnetism, Sliding Ferroelectricity, and Magneto-Optical Kerr Effect in Multiferroic Bilayers

Antiferromagnetic (AFM) materials provide a platform to couple altermagnetic (AM) spin-splitting with the magneto-optical Kerr effect (MOKE), offering potential for next-generation quantum technologies. In this work, first-principles calculations, symmetry analysis, and kp modeling are employed to show that interlayer sliding in AFM multiferroic bilayers enables control of electronic, magnetic, and magneto-optical properties. This study reveals an intriguing dimension-driven AM crossover: the 2D paraelectric (PE) bilayer exhibits spin-degenerate bands protected by the [C2||Mc] spin-space symmetry, whereas the 3D counterpart manifests AM spin-splitting along kz \neq 0 paths. Furthermore, interlayer sliding breaks this Mc symmetry and stabilizes a ferroelectric (FE) state with compensated ferrimagnetism, where the Zeeman-like field is responsible for the nonrelativistic spin-splitting. In the FE phase, spin-orbit coupling (SOC) lifts accidental degeneracies and produces `alternating' spin-polarized bands through the interplay of Zeeman and Rashba effects. Crucially, spin polarization, ferrovalley polarization, and the Kerr angle can all be reversed by switching either sliding ferroelectricity or the Neel vector. Our findings reveal the rich coupling among electronic, magnetic, and optical orders in sliding multiferroics, illustrating new prospects for ultralow-power spintronic and optoelectronic devices.

cond-mat.mtrl-sci↗
arXiv2026

Lead-free piezoelectric perovskites for arterial-pulse e-skin: from configurational complexity to equivariant machine-learning potentials

Continuous, non-invasive monitoring of the arterial pulse is a clinical priority for cardiovascular disease, the leading cause of global mortality. Flexible piezoelectric electronic skins can transduce the 1-10 kPa pressure wave into a self-powered voltage, but the best-performing piezoceramics are lead-based, and their toxicity is incompatible with skin contact and with tightening RoHS/REACH regulation. Among lead-free alternatives, the BaTiO$3$-based solid solution BZT-BCT reaches $d{33} \approx 620$ pC/N near its tricritical morphotropic phase boundary, rivalling soft PZT while remaining biocompatible. Exploiting this in a wearable confronts a sensitivity-flexibility paradox and three computational walls: the combinatorial explosion of atomic configurations in a disordered solid solution, the band-gap error of affordable density-functional approximations, which corrupts leakage and insulation estimates, and the 0 K nature of standard calculations against a 310 K operating temperature. We review lead-free piezoelectrics, morphotropic-boundary physics and fabricated flexible devices, then argue that equivariant machine-learning interatomic potentials --- coupled to a tiered functional hierarchy and finite-temperature lattice dynamics --- can survey the full configurational ensemble at body temperature and close the gap to a clinically viable lead-free pulse sensor.

cond-mat.mtrl-sci↗