A total of nine solid-solution Zintl phases in the Ca 2−x RE x CdSb 2 (RE = Yb, Eu; 0.11(1) ≤ x ≤ 1.36(2)) system with two types of cationic mixtures have been synthesized by the Pb metal-flux method, and their crystal structures have been characterized by powder and single-crystal X-ray diffraction (PXRD and SXRD) measurements. In particular, a series of solid-solution Ca 2−x Yb x CdSb 2 (0.43(2) ≤ x ≤ 1.36(2)) compounds showed a phase-transition from the Ca 2 CdSb 2 -type to the Yb 2 CdSb 2 -type structure depending upon the Ca 2+ /Yb 2+ mixedratio. These two structure types contained nearly identical anionic structural moieties, but their spatial arrangements were distinctive. On the other hand, the three compounds in the Ca 2−x Eu x CdSb 2 (0.11(1) ≤ x ≤ 1.04(2)) system crystallized only in the Yb 2 CdSb 2type phase regardless of the Eu amounts. The observed phase-transition in the Ca 2−x Yb x CdSb 2 system can be attributed to the two kinds of stacking sequence of the octahedral [(Ca/Yb)Sb 6 ] building block and the resultant interatomic interactions between two neighboring Ca 2+ /Yb 2+ mixed-sites in two distinctive title structure types. Moreover, according to SXRD refinements, two types of mixed-cations of Ca 2+ /Yb 2+ or Ca 2+ /Eu 2+ showed noticeable site-preferences between two available atomic sites. To understand the driving force for this phase-transition and the origin of the cationic site-preference, a series of DFT calculations using the TB-LMTO-ASA method were conducted, and the resultant DOS, COHP, and ELF diagrams were thoroughly interrogated. In particular, the COHP analysis successfully supported that the observed phase-transition was triggered by the energetically unfavorable shorter (Ca/Yb)1−(Ca/Yb)1 interaction in the Yb-rich Ca 2 CdSb 2 -type phase. Moreover, the cationic site-preference in the Ca 2−x Yb x CdSb 2 system can be interpreted by the electronic-factor criterion based on the Q value of each site, whereas that in the Ca 2−x Eu x CdSb 2 system can be justified by the size-factor criterion on the basis of the size of cationic elements. The chemical compositions and the appearance of selected single-crystals were analyzed by EDS and SEM, and the thermal stability of a sample was also checked by TGA analysis.
The Zintl phase solid-solution CaYbSbGe (0 ≤ x ≤ 9; 0 ≤ y ≤ 3; 0 ≤ z ≤ 3) system with the cationic/anionic multisubstitution has been synthesized by molten Sn metal flux and arc-melting methods. The crystal structure of the nine title compounds were characterized by both powder and single-crystal X-ray diffractions and adopted the HoGe-type structure with the tetragonal space group I4/mmm (Z = 4, Pearson Code tI84). The overall isotypic structure of the nine title compounds can be illustrated as an assembly of three different types of cationic polyhedra sharing faces with their neighboring polyhedra and the three-dimensional cage-shaped anionic frameworks consisting of the dumbbell-shaped Sb units and the square-shaped Sb or (Sb/Ge) units. During the multisubstitution trials, interestingly, we observed a metal-to-semiconductor transition as the Ca and Ge contents increased in the title system from YbSb to CaYbSbGe (nominal compositions) on the basis of a series of thermoelectric property measurements. This phenomenon can be elucidated by the suppression of a bipolar conduction of holes and electrons via an extra hole-carrier doping. The tight-binding linear muffin-tin orbital calculations using four hypothetical structural models nicely proved that the size of a pseudogap and the magnitude of the density of states at the Fermi level are significantly influenced by substituting elements as well as their atomic sites in a unit cell. The observed particular cationic/anionic site preferences, the historically known abnormalities of atomic displacement parameters, and the occupation deficiencies of particular atomic sites are further rationalized by the QVAL value criterion on the basis of the theoretical calculations. The results of SEM, EDS, and TGA analyses are also provided.
Four novel ternary Zintl phase compounds belonging to the Ca11–x RE x Sb10–y (RE = La, Ce, Nd, Sm; 0.18(4) ≤ x ≤ 0.43(2), 0.14(1) ≤ y ≤ 0.41(1)) system have been synthesized by arc-melting, and the Ho11Ge10-type crystal structure has been characterized for the isotypic title compounds by both powder and single crystal X-ray diffraction analyses. The intrinsically complex crystal structure is viewed as an assembly of the three distinctively shaped cationic polyhedra built from either seven or nine cations and the anionic frameworks constructed by the “dumbbell-shaped” Sb2 and the “square-shaped” Sb4 moieties. All of the four trivalent rare-earth metals were successfully introduced as n-type dopants to partially substitute divalent Ca atoms in the parental compound Ca11Sb10, which resulted in generating two or three Ca2+/RE3+ mixed-cationic sites. In particular, during these substitutions, we observed a unique site preference of Ca2+ and RE3+ among four available cationic sites, where the rare-earth metals with the higher electronegativities than Ca occupied particular atomic sites having the higher Q values. This type of site preference was conclusively explained by theoretical investigations using the tight-binding linear muffin-tin orbital method. Despite the successful n-type doping, the increased electrical conductivities σ and the decreased Seebeck coefficients S of Ca10.75(3)Nd0.25Sb9.82(1) and Ca10.82(4)Sm0.18Sb9.86(1) compared to those of Ca11Sb10 still presented the p-type rather than n-type characters. These unexpected behaviors should be attributed to ca. 7–20% of Sb3 deficiencies found at the “square-site” (Wyckoff 8h), which spontaneously occurred to reduce an energetically unfavorable antibonding character of the interatomic interaction between two Sb3 atoms at the square-site. Total and partial density of states of a hypothetical structural model Ca10.5Nd0.5Sb10, an SEM image of single crystals of Ca10.57(2)La0.43Sb9.59(1), and a TGA result of Ca10.82(4)Sm0.18Sb9.86(1) are also provided.
Two novel bismuth oxyfluoride nitrates, Bi2OF3(NO3) and Bi6O6F5(NO3), have been synthesized via hydrothermal reactions. Whereas Bi2OF3(NO3) crystallizes in the centrosymmetric (CS) hexagonal space group, P63/m, Bi6O6F5(NO3) crystallizes in the polar noncentrosymmetric (NCS) trigonal space group, R3. The backbones of the title compounds reveal double layered structures composed of asymmetric BiF3(O/F)3 or BiO3F2 polyhedra and NO3 trigonal planar groups. The diffuse reflectance spectra indicate that Bi2OF3(NO3) and Bi6O6F5(NO3) contain wide band gaps of 3.5 and 4.0 eV, respectively. Powder second-harmonic generation (SHG) measurements suggest that NCS Bi6O6F5(NO3) is Type-I phase-matchable and has a large SHG response of ca. 3 times that of KH2PO4 (KDP). Electron localization function (ELF) analysis indicates that the large SHG efficiency of Bi6O6F5(NO3) is attributed to the synergistic effect of the alignment of NO3 – trigonal planar groups and strong interactions between highly polarizable lone pair electrons on Bi3+ and π-delocalized electrons in NO3 – groups. Bi2OF3(NO3) also exhibits a very good photocatalytic degradation efficiency of Rhodamine B (RhB) under the UV light irradiation.
Five novel Zintl phases belonging to the ternary Ca11–x RE x Sb10–y (RE = Tb, Dy, Ho, Er, Tm; 0.35(6) ≤ x ≤ 0.41(4), 0.27(1) ≤ y ≤ 0.51(2)) system were successfully synthesized by the arc-melting method, and their crystal structures were carefully determined by using powder and single-crystal X-ray diffraction (PXRD and SXRD) analyses. Large amounts of nicely grown plate-shaped single crystals were easily obtained using this relatively facile synthetic method, and all title compounds adopted the tetragonal I4/mmm space group with Z = 4 and a Pearson code of tI84. This isotypic Ho11Ge10-type structure of the title compounds can be illustrated as a combination of (1) three different types of cationic coordination polyhedra centered by three isolated Sb atoms and (2) three-dimensional (3D) anionic frameworks formed by two different types of Sb atoms. These ternary compounds successfully accommodated the n-type RE3+ dopants at the Ca2 (Wyckoff 16n) site of the parental binary Ca11Sb10, and this particular site preference can be elucidated by the substituting cationic size and the volume of the provided cationic site. A series of theoretical calculations were also performed using the hypothetical model Ca10.5Ho0.5Sb10 by the tight-binding linear muffin-tin orbital (TB-LMTO) method. Thorough analyses of crystal orbital Hamilton population (COHP) curves proved that the antibonding character of the Sb3–Sb3 bond induced the observed deficiencies of the square Sb4 sites. Magnetic property measurements performed for three title compounds, Ca10.59(4)Dy0.41Sb9.71(1), Ca10.61(3)Ho0.39Sb9.69(1), and Ca10.64(3)Er0.36Sb9.73(1), indicated the antiferromagnetic (AFM) interactions of RE elements at relatively low temperatures with paramagnetic Curie temperatures of −2.24, −2.80, and −2.98 K, respectively.
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