RESUMO
SrAl2Si2 crystallizes into either a semimetallic, CaAl2Si2-type, α phase or a superconducting, BaZn2P2-type, ß phase. We explore possible αâPc,â¡Tcß 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-ß 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 ß 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 ß 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.
RESUMO
We follow the evolution of the electronic properties of the titled homologous series when n as well as the atomic type of A and M are varied where for n = 1, A = Ca, Sr and M = Rh, Ir while for n = 3, A = Ca, Sr and M = Rh. The crystal structure of n = 1 members is known to be CaRh2B2-type (Fddd), while that of n = 3 is Ca3Rh8B6-type (Fmmm); the latter can be visualized as a stacking of structural fragments from AM3B2 (P6/mmm) and AM2B2. The metallic properties of the n = 1 and 3 members are distinctly different: on the one hand, the n = 1 members are characterized by a linear coefficient of the electronic specific heat γ ≈ 3 mJ mol-1 K-2, a Debye temperature θD ≈ 300 K, a normal conductivity down to 2 K and a relatively strong linear magnetoresistivity for fields up to 150 kOe. The n = 3 family, on the other hand, exhibits γ ≈ 18 mJ mol-1 K-2, θD ≈ 330 K, a weak linear magnetoresistivity and an onset of superconductivity (for Ca3Rh8B6, Tc = 4.0 K and Hc2 = 14.5 kOe, while for Sr3Rh8 B6, Tc = 3.4 K and Hc2 ≈ 4.0 kOe). These remarkable differences are consistent with the findings of the electronic band structures and density of state (DOS) calculations. In particular, satisfactory agreement between the measured and calculated γ was obtained. Furthermore, the Fermi level, EF, of Ca3Rh8B6 lies at almost the top of a pronounced local DOS peak, while that of CaRh2B2 lies at a local valley: this is the main reason behind the differences between the, e.g., superconducting properties. Finally, although all atoms contribute to the DOS at EF, the contribution of the Rh atoms is the strongest.
