According to an earlier Abrikosov model, a positive, nonsaturating, linear magnetoresistivity (LMR) is expected in clean, low-carrier-density metals when measured at very low temperatures and under very high magnetic fields. Recently, a vast class of materials were shown to exhibit extraordinary high LMR but at conditions that deviate sharply from the above-mentioned Abrikosov-type conditions. Such deviations are often considered within either classical Parish-Littlewood scenario of random-conductivity network or within a quantum scenario of small-effective mass or low carriers at tiny pockets neighboring the Fermi surface. This work reports on a manifestation of novel example of a robust, but moderate, LMR up to ∼100 K in the diamagnetic, layered, compensated, semimetallic CaAl2Si2. We carried out extensive and systematic characterization of baric and thermal evolution of LMR together with first-principles electronic structure calculations based on density functional theory. Our analyses revealed strong correlations among the main parameters of LMR and, in addition, a presence of various transition/crossover events based on which a P − T phase diagram was constructed. We discuss whether CaAl2Si2 can be classified as a quantum Abrikosov or classical Parish-Littlewood LMR system.

We investigated the pressure dependence of the crystal structure of CaAl2Si2 by means of ab initio calculations and room-temperature synchrotron x-ray powder diffraction. Ab initio calculations reproduce satisfactorily the experimentally observed pressure-dependent structural evolution up to 3 GPa where the title system remains in the trigonal phase. In the pressure range 3–8 GPa, pressure evolution of the calculated in-plane lattice parameters is steeper than the observed. Ab initio calculations predict a structural phase transition to a tetragonal phase ( to I4/mmm) near 7.5 GPa for zero (or room) temperature. Temperature effects are included through calculation of vibrational properties (phonon spectra). These calculations confirm that both phases are either globally or locally stable (metastable) and allow for the construction of a P − T phase diagram for this system. However, our experiments show no sign of such a transition up to 12 GPa. Such a discrepancy can be explained if one considers the trigonal () structure to be metastable above the critical pressure, but is separated from the predicted tetragonal (I4/mmm) structure by a relatively high energy barrier. The applied pressure alone may not be able to surpass the energy-barrier; rather a joint high-pressure and high-temperature (HPHT) treatment may lead to it. However, empirical verification of such a hypothetical transition may be hampered by the chemistry of CaAl2Si2 system which shows tendency to decompose peritectically into Ca2Al3Si4 and aluminum under HPHT treatment.

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SrAl 2 Si 2 crystallizes into either a semimetallic, CaAl 2 Si 2 -type, α phase or a superconducting, BaZn 2 P 2 -type, β phase. We explore possible α Pc,Tc − −− → β transformations by employing pressure-and temperature-dependent free-energy calculations, vibrational spectra calculations, and room-temperature synchrotron X-ray powder diffraction (XRPD) measurements up to 14 GPa using diamond anvil cell. Our theoretical and empirical analyses together with all baric and thermal reported events on both phases allow us to construct a preliminary P -T diagram of transformations. Our calculations show a relatively low critical pressure for the α to β transition (4.9 GPa at 0 K, 5.0 GPa at 300 K and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure-low-temperature α phase is separated from metastable β phase by a relatively high activation barrier. This analysis is supported by our XRPD data at ambient temperature and P ≤ 14 GPa which shows an absence of β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl 2 Si 2 -dimorphs or total chemical substitution of Ca by Sr in isomorphous α CaAl 2 Si 2 ; empirically, manifestation of such topology modification is evident when comparing the evolution of (magneto-)transport properties of members of SrAl 2 Si 2 -dimorphs and α isomorphs.

SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, α phase or a superconducting, BaZn2P2-type, β phase. We explore possible transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectral calculations, and room-temperature synchrotron powder X-ray diffraction (PXRD) measurements up to 14 GPa using a diamond anvil cell. Our theoretical and empirical analyses together with all reported baric and thermal events on both phases allow us to construct a preliminary P–T diagram of transformations. Our calculations show a relatively low critical pressure for the α-to-β transition (4.9 GPa at 0 K, 5.0 GPa at 300 K, and 5.3 GPa at 900 K); nevertheless, our nonequilibrium analysis indicates that the low-pressure low-temperature α phase is separated from a metastable β phase by a relatively high activation barrier. This analysis is supported by our PXRD data at ambient temperature and P ≤ 14 GPa, which shows an absence of the β phase even after a compression involving three times the critical pressure. Finally, we briefly consider the change in the Fermi surface topology when atomic rearrangement takes place via either transformations among SrAl2Si2 dimorphs or total chemical substitution of Ca by Sr in the isomorphous CaAl2Si2 α phase; empirically, the manifestation of such a topology modification is evident upon comparison of the evolution of the (magneto)­transport properties of members of SrAl2Si2 dimorphs and α isomorphs.