Magnetic and structural transitions in ${\text{La}}_{1-x}{A}_x{\text{CoO}}_3$ ($A=\text{Ca}$, Sr, and Ba)

We report thermal-expansion, lattice-constant, and specific-heat data of the series ${\text{La}}_{1-x}{A}_x{\text{CoO}}_3$ for $0\le x\le 0.30$ with $A=\text{Ca}$, Sr, and Ba. For the undoped compound ${\text{LaCoO}}_3$, the thermal-expansion coefficient $\alpha \left(T\right)$ exhibits a pronounced maximum around $T=50\text{K}$ caused by a temperature-driven spin-state transition from a low-spin state of the ${\text{Co}}^{3+}$ ions at low temperatures toward a higher spin state at higher temperatures. The partial substitution of the ${\text{La}}^{3+}$ ions by divalent ${\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, or ${\text{Ba}}^{2+}$ ions causes drastic changes in the macroscopic properties of ${\text{LaCoO}}_3$. The large maximum in $\alpha \left(T\right)$ is suppressed and completely vanishes for $x\gtrsim 0.125$. For $A=\text{Ca}$ three different anomalies develop in $\alpha \left(T\right)$ with further increasing $x$, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data, we identify several phase transitions as a function of the doping concentration $x$ and temperature. From these data we propose an extended phase diagram for ${\text{La}}_{1-x}{\text{Ca}}_x{\text{CoO}}_3$.

Figure 1

(Color online) Thermal-expansion coefficient $\alpha$ vs $T$ of (a) ${\text{La}}_{1-x}{\text{Sr}}_x{\text{CoO}}_3$ and (b) ${\text{La}}_{1-x}{\text{Ba}}_x{\text{CoO}}_3$. The arrows signal the direction of increasing $x$. The curves for $x=0.08$ and 0.125 in panel (a) and for $x=0.15$ and 0.2 in panel (b), respectively, lie almost on top of each other.

Figure 2

(Color online) Specific heat displayed as $c_{\mathrm{p}}/T$ vs $T$ of (a) ${\text{La}}_{1-x}{\text{Sr}}_x{\text{CoO}}_3$ and (b) ${\text{La}}_{1-x}{\text{Ba}}_x{\text{CoO}}_3$. For clarity, the data for different $x$ are shifted by $+0.05\text{J}/\text{mol}{\text{K}}^2$ with respect to each other. The black circles denote either the spin-glass freezing temperatures or the onset temperatures of ferromagnetic order given in Ref. 21. The arrows signal the direction of increasing $x$.

Figure 3

(Color online) Thermal-expansion coefficient $\alpha$ vs $T$ of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{CoO}}_3$: in panel (a) $\alpha \left(T\right)$ for the low-doping region $x\le 0.125$ and in panel (b) for the high-doping region $0.23\le x\le 0.27$ is given. For comparison, the data set for $x=0.125$ is shown in both panels. The ferromagnetic transition temperatures from Ref. 21 are denoted by black circles. The arrow in panel (a) signals the direction of increasing $x$.

Figure 4

(Color online) Thermal-expansion coefficient $\alpha$ vs $T$ of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{CoO}}_3$: in panel (a) $\alpha \left(T\right)$ for $0.125\le x\le 0.19$ and in panel (b) for $0.19\le x\le 0.21$ is given. For comparison, the data sets for $x=0.125$ and 0.19 are shown in both panels. The ferromagnetic transition temperatures from Ref. 21 are denoted by black circles.

Figure 5

(Color online) Structural data of ${\text{La}}_{0.85}{\text{Ca}}_{0.15}{\text{CoO}}_3$ as a function of temperature measured upon cooling. In panels (a)–(c), the orthorhombic $\left(Pbnm\right)$ and in (d) and (e) the hexagonal $\left(R\overline{3}c\right)$ lattice constants are shown. Panel (f) shows the average volume per formula unit. In (g) the orthorhombic and rhombohedral phase fractions measured with decreasing (closed symbols) and increasing (open symbols) temperatures are shown. The thin gray line in panels (a)–(f) marks the temperature $\left(\sim 144\text{K}\right)$ of equal amounts of both phases in the measurements with decreasing temperature. The second gray line in panel (g) marks the corresponding temperature $\left(\sim 163\text{K}\right)$ in the subsequent measurement with increasing temperature.

Figure 6

(Color online) Specific heat displayed as $c_{\mathrm{p}}/T$ vs $T$ of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{CoO}}_3$: for clarity the data for different $x$ are shifted by $+0.1\text{J}/\text{mol}{\text{K}}^2$ with respect to each other. In panel (a) the curves for the doping region $0\le x\le 0.15$, in (b) $0.15\le 0.19$, and in (c) $0.19\le 0.3$ are given. The black circles denote the onset temperatures of ferromagnetic order from Ref. 21. The blue stars label the LTM and the green diamonds the HTM occurring in the thermal-expansion data.

Figure 7

(Color online) Phase diagram of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{CoO}}_3$ based on the transition temperatures given in Table obtained by magnetization (Ref. 21), thermal-expansion, specific-heat, and x-ray diffraction measurements (this work). PM denotes the paramagnetic and FM the quasiferromagnetic phase, respectively. The transition from an insulator to a “metal” takes place only gradually in this series (compare Ref. 21). The lines are guides for the eyes. Please note that the structural transition is of first order. The phase boundary shown here is based on the measurements upon warming. The respective phase boundary obtained on cooling lies lower in temperature. Moreover, there is a rather large region of coexisting $R\overline{3}c$ and $Pbnm$ phases (see text).