Pressure/temperature/substitution-induced melting of A-site charge disproportionation in ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$ $(0⩽x⩽0.5)$

Metal-insulator transitions strongly coupled with lattice were found in ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$. Synchrotron x-ray powder diffraction revealed that pressure ($P\sim 3\mathrm{GPa}$, $T=300\mathrm{K}$), temperature ($T\sim 340\mathrm{K}$, $x=0.05$), and La substitution ($x\sim 0.075$, $T=300\mathrm{K}$) caused the similar structural change from a triclinic (insulating) to an orthorhombic (metallic) symmetry, suggesting melting of the A-site charge disproportionation. Comparing crystal structure and physical properties with the other $A{\mathrm{NiO}}_3$ series, an electronic state of the metallic phase can be described as [$A^{3+}{\underset{̱}{L}}^{\delta }$, ${\mathrm{Ni}}^{2+}{\underset{̱}{L}}^{1-\delta }$], where a ligand-hole $\underset{̱}{L}$ contributes to a conductivity. We depicted a schematic $P\text{−}T$ phase diagram of ${\mathrm{BiNiO}}_3$ including a critical point (3 GPa, 300 K) and an inhomogeneous region, which implies universality of ligand-hole dynamics in $A{\mathrm{NiO}}_3$ ($A=\mathrm{Bi}$, Pr, Nd… ).

Figure 1

XRD patterns of ${\mathrm{BiNiO}}_3$ at (a) 0.5, 1.1, 2.1, 3.0, 3.7, and 4.8 GPa with $\lambda =0.4966\mathrm{\AA }$, (b) ${\mathrm{Bi}}_{0.95}{\mathrm{La}}_{0.05}{\mathrm{NiO}}_3$ at 100, 200, 300, 320, 330, 340, 360, 380, and 400 K with $\lambda =0.776\mathrm{\AA }$, and (c) ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$ ($x=0$, 0.05, 0.075, 0.1, 0.2, and 0.5) with Cu ${K}\alpha$ at room temperature, traversing the phase transition between triclinic (with CD) and orthorhombic (without CD) perovskite. An impurity phase was indicated by an asterisk.

Figure 2

Measured $(+)$, calculated (line), and differential (bottom line) SXRD patterns for ${\mathrm{Bi}}_{0.95}{\mathrm{La}}_{0.05}{\mathrm{NiO}}_3$ at (a) 400 K and (b) 100 K. The ticks indicate the positions of the reflections: upper in (a), orthorhombic; lower in (a) and (b), triclinic. The mass fraction of a triclinic phase at 400 K is 6%.

Figure 3

Pressure dependence of (a) lattice parameters, (b) unit-cell angles, (c) unit-cell volume $(Z=4)$, and (d) resistivity of ${\mathrm{BiNiO}}_3$ at room temperature $(T=300\mathrm{K})$. In (a)–(c), the data were collected with increasing pressure. The open and closed symbols signify the AP and HP phases, respectively. The dashed lines in (b) are guides to the eyes.

Figure 4

Temperature dependence of (a) lattice parameters, (b) unit-cell angles, (c) unit-cell volume $(Z=4)$, and resistivity of ${\mathrm{Bi}}_{0.95}{\mathrm{La}}_{0.05}{\mathrm{NiO}}_3$. In (a)–(c), the data were collected on heating. The open and filled symbols signify the LT and HT phases, respectively.

Figure 5

La-content, $x$, dependence of (a) lattice parameters, (b) unit-cell angles, (c) unit-cell volume $(Z=4)$, and resistivity of ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$ at room temperature $(T=300\mathrm{K})$. The dashed lines in (b) are guides to the eyes.

Figure 6

(Color online) Temperature dependence of (a) resistivity, (b) Seebeck coefficient, (c) magnetic susceptibility, and (d) inverse magnetic susceptibility of ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$.

Figure 7

Magnitude of the orthorhombic distortion, $b∕a$, of $A{\mathrm{NiO}}_3$ as a function of the mean ionic radii of A-site ion $r_{\mathrm{A}}$ (a), and that of ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$ as a function of La content $x$ and pressure (b). The ionic radii for trivalent ions with eight coordination were adopted for $r_{\mathrm{A}}$.

Figure 8

Schematic phase diagram of ${\mathrm{Bi}}_{1-x}{\mathrm{La}}_x{\mathrm{NiO}}_3$ as functions of pressure, La content $x$, and temperature.