Thermal expansion and elastic properties of mullite-type Bi₂Ga₄O₉ and Bi₂Fe₄O₉ single crystals

Abstract

Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi 2 Ga 4 O 9 and Bi 2 Fe 4 O 9 between room temperature and about 1 200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1 280 K using high-resolution dilatometry and X-ray powder diffraction. for Bi 2 Fe 4 O 9 .I n both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the c ij shows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge-and corner-linked coordination polyhedra.

Thermal expansion of mullite-type Bi2Al4O9: A study by X-ray diffraction, vibrational spectroscopy and density functional theory

“…Alike mullite, the thermal expansion behavior violates the empirical rule (the lowest thermal expansion occurs to the elastically stiffest crystal direction). This discrepancy was explained in terms of pure geometric considerations [27][28][29], however, the underlying microscopic reason is still unclear. The phonon density of states (PDOS) and its partial components from each atom in the asymmetric unit (Bi, Al, O) are shown in Fig.…”

“…3; almost similar behaviors of the corresponding ones in Bi 2 Ga 4 O 9 [25]. For a comparison, the axial bulk moduli for Bi 2 Ga 4 O 9 (K a ¼230.4(4)GPa, K b ¼309.5(5) and K c ¼446(2)GPa, in the crystallographic a-b-and c-direction) and Bi 2 Fe 4 O 9 (K a ¼ 163.6(1)GPa, K b ¼303.8(5)GPa and K c ¼ 431.6(1)GPa) were calculated [26] from the observed elastic constants [27,28]. López-de-la-Torre et al [29] observed the similar trend for Bi 2 Al 4 O 9 and Bi 2 Mn 4 O 10 compounds, showing that the linear incompressibities of these isotypic materials follow the order as K c 4K b 4K a .…”

Influence of the Bi 6s 2 lone electron pair on elastic properties of monoclinic Bi4B2O9

“…However, a comprehensive study of mullite-type Bi 2 M 4 O 9 (M = Al, Ga, Fe) showed that in these compounds the polarized LEP is important for the stabilisation of the crystal structure (see [13], and references therein). On the other hand, the elastically soft LEP leads to a distinct reduction of their mean elastic stiffness [14].…”

Effect of Al3+ substitution on the structural, magnetic, and electric properties in multiferroic Bi2Fe4O9 ceramics

“…The structure analysis shows that the mullite-type structures are characterized by one dimensional chains of edge sharing MO 6 octahedra, and M 2 O 7 double tetrahedral units and BiO 6 E groups (where E refers to a 6s 2 lone electron pair) interconnect the octahedral chains in an alternating ordered manner along the caxis [4]. From the perspective of application, the phase should be stable up to a temperature of about 1200 K without showing any phase transition or decomposition.…”

“…Gesing et al have systematically investigated the thermal expansion, stability, and elasticity properties in (Bi 1 À x Sr x ) 2 (M 1 1 À y M 2 y ) 4 O 9 À x and Bi 2 M 4 O 9 (M¼Al 3 þ , Ga 3 þ , and Fe 3 þ ). [5][6][7][8] Additionally, the extensive formation of Bi 2 M 1 4 O 9 -Bi 2 M 2 4 O 9 solid solutions provides an additional feature for the magnetic, electrical, and other materials properties which often depend on the distribution of M 1 and M 2 cations on the available octahedral and tetrahedral sites, and Weber et al have used 57 Fe Mössbauer spectroscopy of Bi 2 (Fe x M 1 À x ) 4 O 9 (M ¼Al 3 þ , Ga 3 þ ) to investigate the distribution of Fe 3 þ and Al 3 þ /Ga 3 þ in the octahedral and tetrahedral sites [9,10].…”