Crystal Cluster Growth and Physical Properties of the EuSbSe₃ and EuBiSe₃ Phases

Syntheses of europium metal, selenium powder, and the Sb₂Se₃/Bi₂Se₃ binaries were observed to produce crystal clusters of the EuSbSe₃ and EuBiSe₃ phases. These phases crystallize with the <i>P</i>2₁2₁2₁ space group and can be easily identified based on their growth habits, forming large clusters of needles. Previous literature suggested that their structure is charge-balanced with all europium atoms in the divalent state and one-quarter of the selenium atoms forming trimers. Physical property measurements on a pure sample of EuSbSe₃ revealed typical Arrhenius-type electrical resistivity, being approximately 3 orders of magnitude too large for thermoelectric applications. Electronic structure calculations indicated that both EuSbSe₃ and EuBiSe₃ are narrow-band-gap semiconductors, in good agreement with the electrical resistivity data. The valence and conduction band states near the Fermi level are dominated by the Sb/Bi and Se p states, as expected given their small difference in electronegativity

<i>RE</i>₃Mo₁₄O₃₀ and <i>RE</i>₂Mo₉O₁₉, Two Reduced Rare-Earth Molybdates with Honeycomb-Related Structures (<i>RE</i> = La–Pr)

The previously unreported <i>RE</i>₃Mo₁₄O₃₀ and <i>RE</i>₂Mo₉O₁₉ phases were synthesized in vacuo from rare-earth oxides, molybdenum oxide, and molybdenum metal using halide fluxes at 875–1000 °C. Both phases adopt structures in the triclinic <i>P</i>1̅ space group albeit with several notable differences. The structures display an ordering of layers along the <i>a</i> direction of the unit cell, forming distinct honeycomb-related lattice arrangements composed of MoO₆ octahedra and vacancies. Mo–Mo bonding and clusters are present; the <i>RE</i>₃Mo₁₄O₃₀ structure contains Mo dimers and rhomboid tetramers, while the <i>RE</i>₂Mo₉O₁₉ structure contains rhomboid tetramers and an unusual arrangement of planar tetramers, pentamers, and hexamers. The magnetic measurements found the <i>RE</i>₂Mo₉O₁₉ phases to be simple paramagnets, while La₃Mo₁₄O₃₀ was observed to order antiferro­magnetically at 18 K. Electrical resistivity measurements confirmed all of the samples to behave as nondegenerate semiconductors

Crystal Cluster Growth and Physical Properties of the EuSbSe₃ and EuBiSe₃ Phases

Syntheses of europium metal, selenium powder, and the Sb₂Se₃/Bi₂Se₃ binaries were observed to produce crystal clusters of the EuSbSe₃ and EuBiSe₃ phases. These phases crystallize with the <i>P</i>2₁2₁2₁ space group and can be easily identified based on their growth habits, forming large clusters of needles. Previous literature suggested that their structure is charge-balanced with all europium atoms in the divalent state and one-quarter of the selenium atoms forming trimers. Physical property measurements on a pure sample of EuSbSe₃ revealed typical Arrhenius-type electrical resistivity, being approximately 3 orders of magnitude too large for thermoelectric applications. Electronic structure calculations indicated that both EuSbSe₃ and EuBiSe₃ are narrow-band-gap semiconductors, in good agreement with the electrical resistivity data. The valence and conduction band states near the Fermi level are dominated by the Sb/Bi and Se p states, as expected given their small difference in electronegativity

Synthesis, Crystal Structure, and Electronic Properties of the Tetragonal (RE^IRE^II)₃SbO₃ Phases (RE^I = La, Ce; RE^II = Dy, Ho)

In our efforts to tune the charge transport properties of the recently discovered RE₃SbO₃ phases (RE is a rare earth), we have prepared mixed (RE^IRE^II)₃SbO₃ phases (RE^I = La, Ce; RE^II = Dy, Ho) via high-temperature reactions at 1550 °C or greater. In contrast to monoclinic RE₃SbO₃, the new phases adopt the <i>P</i>4₂/<i>mnm</i> symmetry but have a structural framework similar to that of RE₃SbO₃. The formation of the tetragonal (RE^IRE^II)₃SbO₃ phases is driven by the ordering of the large and small RE atoms on different atomic sites. The La_1.5Dy_1.5SbO₃, La_1.5Ho_1.5SbO₃, and Ce_1.5Ho_1.5SbO₃ samples were subjected to elemental microprobe analysis to verify their compositions and to electrical resistivity measurements to evaluate their thermoelectric potential. The electrical resistivity data indicate the presence of a band gap, which is supported by electronic structure calculations

Synthesis, Crystal Structure, and Electronic Properties of the CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Rare-Earth Metal)

Through high temperature synthesis at 1300 °C and above, our group has discovered and characterized the novel CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Ce–Nd, Sm–Dy for CaRE₃SbO₄, RE = La–Nd, Sm–Dy for Ca₂RE₈Sb₃O₁₀). This result was motivated by the idea of opening a band gap and introducing structural complexity in the rare-earth antimonide framework by incorporation of rare-earth oxide and calcium oxide. The CaRE₃SbO₄ phases adopt the tetragonal <i>I</i>4/<i>m</i> symmetry while the Ca₂RE₈Sb₃O₁₀ ones adopt the monoclinic <i>C</i>2/<i>m</i> symmetry. These structures show many similarities to the other RE–Sb–O phases discovered recently, particularly to the RE₃SbO₃ and RE₈Sb₃O₈ phases, in which a prolonged heat treatment results in one structure converting to another by elongation of the rare-earth oxide slabs. Electrical resistivity measurements yielded semiconducting properties for both series, despite the unbalanced electron count for Ca₂RE₈Sb₃O₁₀ and electronic structure calculations that support metallic-type conduction. This unusual behavior is attributed to Anderson-type localization of Sb p states near the Fermi level, which arises from the highly disordered Sb layers in the structure. This Sb disorder was shown to be tunable with respect to the size of the rare-earth used, improving the electrical resistivity by approximately 1 order of magnitude for each rare-earth in the series

Synthesis, Crystal Structure, and Electronic Properties of the CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Rare-Earth Metal)

Through high temperature synthesis at 1300 °C and above, our group has discovered and characterized the novel CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Ce–Nd, Sm–Dy for CaRE₃SbO₄, RE = La–Nd, Sm–Dy for Ca₂RE₈Sb₃O₁₀). This result was motivated by the idea of opening a band gap and introducing structural complexity in the rare-earth antimonide framework by incorporation of rare-earth oxide and calcium oxide. The CaRE₃SbO₄ phases adopt the tetragonal <i>I</i>4/<i>m</i> symmetry while the Ca₂RE₈Sb₃O₁₀ ones adopt the monoclinic <i>C</i>2/<i>m</i> symmetry. These structures show many similarities to the other RE–Sb–O phases discovered recently, particularly to the RE₃SbO₃ and RE₈Sb₃O₈ phases, in which a prolonged heat treatment results in one structure converting to another by elongation of the rare-earth oxide slabs. Electrical resistivity measurements yielded semiconducting properties for both series, despite the unbalanced electron count for Ca₂RE₈Sb₃O₁₀ and electronic structure calculations that support metallic-type conduction. This unusual behavior is attributed to Anderson-type localization of Sb p states near the Fermi level, which arises from the highly disordered Sb layers in the structure. This Sb disorder was shown to be tunable with respect to the size of the rare-earth used, improving the electrical resistivity by approximately 1 order of magnitude for each rare-earth in the series