Evolution of spin and valence states of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ at high temperature and high pressure

The changes of physical properties occurring at a first-order-like transition among a wide family of $(\mathrm{Pr}{}_{1-y}$$L{n}_y){}_{1-x}\mathrm{Ca}{}_x\mathrm{CoO}{}_3$ compounds (Ln being a lanthanide) are controversially discussed as a transition of $\mathrm{Co}{}^{3+}$ ions from states of low spin (LS) to intermediate spin (IS), or from LS to a mixture of LS and high spin (HS). The debate also includes the nature of the spin state of $\mathrm{Co}{}^{4+}$, as well as the degree of valence shift between $\mathrm{Pr}{}^{3+}$ and $\mathrm{Pr}{}^{4+}$ which accompanies this transition. In the present paper, we investigated the evolution of spin and valence states of Co ions and the valence state of Pr ions in $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$, at high temperature up to 750 K and under pressure up to 36.5 GPa. For this purpose, the data of three spectroscopic techniques were combined: high-resolution Co $K$-edge partial-fluorescence-yield x-ray absorption spectra, Pr $L_2$-edge x-ray absorption spectra, and Co $K$β x-ray emission spectra. The spectral weight transfer of the pre-edge peaks of the Co $K$-edge spectra reveals a continuous redistribution of $3d$ electrons between the $t_{2g}$ and $e_g$ levels of Co ions, reflecting a gradual increase of the average Co spin state upon heating. This is further confirmed by Co $K$β x-ray emission spectra, which show an increase of HS $\mathrm{Co}{}^{3+}$ population when increasing temperature from 300 to 750 K. Applying high pressure at 300 K, the Co $K$β x-ray emission spectra indicate a relatively sharp increase of LS $\mathrm{Co}{}^{3+}$ population below 4 GPa. Pr $L_2$-edge x-ray absorption spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ exhibit a valence-state transition from $\mathrm{Pr}{}^{3+}$ to $\mathrm{Pr}{}^{3.26+}$, occurring predominantly upon applying pressure in the range ∼4 to ∼12 GPa. Owing to the charge balance, increasing pressure in this regime induces complex valence and spin-state transitions within the Co ions. Moreover, at high enough pressure, it is found that the $\mathrm{Co}{}^{4+}$ can also undergo a spin-state transition (from IS to LS), in addition to that affecting the $\mathrm{Co}{}^{3+}$ (HS to LS).

Figure 1

Evolution of the Co $K$β XES spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ with temperature from RT to 750 K. The inset is an enlargement of the $K$β′ satellite region. An arrow indicates the spectral variations with increasing temperature.

Figure 2

Evolution of the Co $K$-edge partial-fluorescence-yield XAS spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ with temperature. The inset shows an enlargement of the pre-edge region. Arrows indicate the spectral variation with increasing temperature. Labels mark the main features discussed in the text.

Figure 3

Evolution of the Pr $L_2$-edge XAS spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ with pressure at 300 K. The data for 12, 16, and 18.4 GPa at RT are compared with the spectrum (red line) recorded for $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ at 10 K/AP.

Figure 4

Pressure dependence (solid squares) and temperature dependence (open circles) of the $\mathrm{Pr}{}^{3+}$ content per f.u. in $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}$CoO at RT and AP, respectively. The data of temperature dependence of the $\mathrm{Pr}{}^{3+}$ content per f.u. below RT are taken from Ref. [27]. The dashed line is only a guide for the eye.

Figure 5

Evolution of the Co $K$β x-ray emission spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ as a function of pressure from RT/AP to RT/36.5 GPa. The inset is an enlargement of the $K$β′ satellite region. An arrow indicates the spectral variation with increasing pressure.

Figure 6

IAD values of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ as a function of applied pressure and temperature. The inset shows the difference spectrum of the Co Kβ emission line between RT/AP and RT/36.5 GPa (red open circles) and the difference spectrum of the emission line between 750 K/AP and RT/36.5 GPa (blue open squares).

Figure 7

Populations of IS $\mathrm{Co}{}^{4+}$, LS $\mathrm{Co}{}^{4+}$, HS $\mathrm{Co}{}^{3+}$, and LS $\mathrm{Co}{}^{3+}$ in $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ (a) as a function of pressure from RT/AP to RT/36.5 GPa, and (b) as a function of temperature from 750 K/AP to 10 K/AP (low-temperature data are taken from Ref. [27]).