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Register a new userStability of vacancy-free crystalline phases of titanium monoxide at high pressure and temperature
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Added by
Nikolay Chtchelkatchev
Publication date
2019
fields
Physics
and research's language is
English
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There have existed for a long time a paradigm that TiO phases at ambient conditions are stable only if structural vacancies are available. Using an evolutionary algorithm, we perform an ab initio search of possible zero-temperature polymorphs of TiO in wide pressure interval. We obtain the Gibbs energy of the competing phases taking into account entropy via quasiharmonic approximation and build the pressure-temperature diagram of the system. We reveal that two vacancy-free hexagonal phases are the most stable at relatively low temperatures in a wide range of pressures. The transition between these phases takes place at 28 GPa. Only above 1290 K at ambient pressure the phases with vacancies (B1-derived) become stable. In particular, the high-pressure hexagonal phase is shown to have unusual electronic properties, with a pronounced pseudo-gap in the electronic spectrum. The comparison of DFT-GGA and GW calculations demonstrates that the account for many-body corrections significantly changes the electronic spectrum near the Fermi energy.
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We present ab initio density-functional study of the noncentrosymmetric B20-type phase of RhGe, which is not found in nature and can be synthesized only at extreme pressures and temperatures. The structural, thermodynamic, electronic, lattice-dynamical, and transport properties of B20-RhGe are calculated, and their evolution with increasing pressure is traced. The temperature dependence of the charge and heat transport properties is evaluated within the semi-classical Boltzmann approach. Using the quasi-harmonic approximation, we determine the range of pressures and temperatures, in which B20-RhGe is stable, and make recommendations for optimizing the synthesis conditions in order to reduce the number of defects that occur in a sample during solidification.
The search and exploration of new materials not found in nature is one of modern trends in pure and applied chemistry. In the present work, we report on experimental and textit{ab initio} density-functional study of the high-pressure-synthesized series of compounds Mn$_{1-x}$(Co,Rh)$_x$Ge. These high-pressure phases remain metastable at normal conditions, therewith they preserve their inherent noncentrosymmetric B20-type structure and chiral magnetism. Of particular interest in these two isovalent systems is the comparative analysis of the effect of $3d$ (Co) and $4d$ (Rh) substitution for Mn, since the $3d$ orbitals are characterized by higher localization and electron interaction than the $4d$ orbitals. The behavior of Mn$_{1-x}$(Co,Rh)$_x$Ge systems is traced as the concentration changes in the range $0 leq x leq 1$. We applied a sensitive experimental and theoretical technique which allowed to refine the shape of the temperature dependencies of magnetic susceptibility $chi(T)$ and thereby provide a new and detailed magnetic phase diagram of Mn$_{1-x}$Co$_x$Ge. It is shown that both systems exhibit a helical magnetic ordering that very strongly depends on the composition $x$. However, the phase diagram of Mn$_{1-x}$Co$_x$Ge differs from that of Mn$_{1-x}$Rh$_x$Ge in that it is characterized by coexistence of two helices in particular regions of concentrations and temperatures.
A study of high pressure solid Te was carried out at room temperature using Raman spectroscopy and Density Functional Theory (DFT) calculations. The analysis of the P-dependence of the experi- mental phonon spectrum reveals the occurrence of phase transitions at 4 GPa and 8 GPa confirming the high-pressure scenario recently proposed. The effects of the incommensurate lattice modulation on the vibrational properties of Te is discussed. DFT calculations agree with present and previous experimental data and show the metallization process at 4 GPa being due to the development of charge-bridges between atoms belonging to adjacent chains. A first-principles study of the stability of the 4 GPa phase is reported and discussed also in the light of the insurgence of lattice modulation.
In the search for MgB2-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (Tc) of 5.5 K. We have extended our previous study of CaB6 [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB[6+x], finding that extra boron further stabilizes the proposed B24 units. Here an explanation is given for the transformation of cP7-CaB6 into the more complex oS56 and tI56 polymorphs at high pressure. The stability of the known metallic tP20 phase of CaB4 at ambient pressure is explained from a crystal structure and chemical bonding point of view. The tP20 structure is shown to destabilize at 19 GPa relative to a semiconducting MgB4-like structure due to chemical pressure from the metal ion. The hypothetical AlB2-type structure of CaB2, previously shown to have favorable superconducting features, is demonstrated here to be unstable at all pressures; two new metallic CaB2 polymorphs with unusual boron networks stabilize at elevated pressures above 8 GPa but are found to have very low critical temperatures (Tc ~1 K). The stability of all structures has been rationalized through comparison with alkaline-earth analogs, emphasizing the importance of the size of the metal ion for the stability of borides. Our study illustrates the inverse correlation between the thermodynamic stability and superconducting properties and the necessity to carefully examine both in the design of new synthesizable superconducting materials.
The phase diagram of oxygen is investigated for pressures from 50 to 130~GPa and temperatures up 1200 K using first principles theory. A metallic molecular structure with the $P6_3/mmc$ symmetry ($eta^{}$ phase) is determined to be thermodynamically stable in this pressure range at elevated temperatures above the $epsilon$(${O_8}$) phase. Long-standing disagreements between theory and experiment for the stability of $epsilon$(${O_8}$), its metallic character, and the transition pressure to the $zeta$ oxygen phase are resolved. Crucial for obtaining these results are the inclusion of anharmonic lattice dynamics effects and accurate calculations of exchange interactions in the presence of thermal disorder.
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