Structural evolution across the metal-insulator transition of strongly distorted Lu1−xScxNiO3 perovskites (x = 0, 0.1, 0.2)

RNiO3 perovskites have been described to present thermally driven metal-insulator transitions (at TMI) as a function of the rare-earth ion size (R = Pr to Lu). Aiming to extend the stability range of RNiO3 for smaller R3+ ions, we prepared Lu1−xScxNiO3 (x = 0, 0.1, 0.2) perovskites, being Sc3+ ions substantially smaller than Lu3+, by using a multi-anvil high-pressure synthesis device at 10 GPa. We have studied the structural evolution of Lu0.9Sc0.1NiO3 by synchrotron x-ray diffraction (SXRD) from room temperature to 350 °C. The symmetry of the lattice evolves from monoclinic (P2 1 /n) to orthorhombic (Pbnm) upon heating across TMI (≈320 °C), with the existence of two chemically and crystallographically distinct nickel sites in the insulating, monoclinic regime, whereas the metallic phase has a single NiO6 environment. A simultaneous structural and electronic transition implies an abrupt evolution of the lattice parameters and size of the NiO6 octahedra upon entering the metallic regime, leading to the merging of the disproportionated Ni-O bond lengths. The magnetic properties correspond to the establishment of antiferromagnetic correlations at the Ni sublattice; a decrease of the TN ordering temperature from 122 K (x = 0) to 113 K (x = 0.2) is observed as the Sc content increases, which is concomitant with a more distorted perovskite structure.


Introduction
Rare-earth nickelates RNiO 3 have attracted the interest of researchers over the last three decades aiming to establish the relationships between structure and observed physical properties. While initially described in 1971 by Demazeau et al [1] the materials underwent a resurgence of interest after the discovery of a thermally driven metal-insulator (MI) transition depending on the rare-earth ion size [2][3][4][5][6]. In the case of small rare-earths (R=Ho-Lu, Y) it was subsequently shown that the symmetry of the lattice evolves from monoclinic (P2 1 /n) to orthorhombic (Pbnm) upon heating across T MI [7][8][9][10][11], with the presence of two nickel sites from the chemical and crystallographical point of view in the insulating, monoclinic regime, whereas the metallic phase has a single NiO 6 environment. The phenomenology involving MI transitions is peculiar of electron-correlated systems, and therefore RNiO 3 oxides have always been taken as paradigmatic examples where to investigate fundamental problems of condensed matter [12]. Moreover, their properties find applications as multiferroic oxides [13] superlattices [14][15][16], fuel cells [17] memory devices [18] or bio-electronic interfaces [19]. Negative magnetorresistance has been described in thin films of NdNiO 3 [20,21], and superconductivity below T C =9-15 K for infinite-layer derivatives of composition Nd 0.8 Sr 0.2 NiO 2 [22]. First-principles electronic structure calculations show that charge ordered rare-earth nickelates of the type RNiO 3  Nd) are multiferroic with very large magnetically-induced ferroelectric (FE) polarizations [13]; multiferroicity has also been described in transition-metal perovskites derived from RNiO 3 , like R 2 NiMnO 6 [23,24].
Since the first diffraction studies across the metallic-insulator transition [3][4][5], the observed symmetry reduction in the insulating regime [7][8][9] was ascribed to a charge disproportionation effect, or uncomplete charge-ordering phenomenon, where Ni 3+ (metallic regime) evolves into Ni 3+δ and Ni 3-δ (insulating regime) located in two differently-sized NiO 6 octahedra. It is owing to the asymmetric distribution of two distinct Ni1 and Ni2 atoms coordinated in octahedra with distinct Ni-O distances [25][26][27]. Alternatively, the structural distortion has been interpreted as the result of a bond disproportionation: the d 8 electronic configuration is preserved in Ni atoms, whereas a positive charge is segregated at the oxygen positions, according to Ni-3d and O-2p hybridization. A configuration 3d 8 L 1 →3d 8 +3d 8 L 2 is ascribed to the disproportionated state, according to spectroscopic studies [26,28,29].
The stabilization of these materials containing Ni 3+ becomes more difficult with the reduction in size of the rare-earth cation along the lanthanide series. After the successful preparation of RNiO 3 for the smallest rareearths R=Er, Tm, Yb, Lu, at moderate pressures (2 GPa), we were able to study by neutron powder diffraction (NPD) the crystal evolution across T MI (for instance, [25] for Yb and Tm, or [10] for LuNiO 3 ). Aiming to extend the stability range of RNiO 3 for even smaller R 3+ ions, we proposed the preparation of Lu 1−x Sc x NiO 3 perovskites, being Sc 3+ ion (VIII: 0.870 Å) substantially smaller than Lu 3+ (VIII: 0.977 Å). This involves a decrease of the tolerance factor, with an additional tilting effect of the octahedra and implying even more severe preparation conditions. This was achieved in a multianvil high-pressure synthesis device, able to reach over 10 GPa.
Moreover, these compounds exhibit antiferromagnetic magnetic structures at low temperature, arising from competing interactions at the Ni sublattice, with Néel temperatures identical to T MI for larger R cations (R=Pr, Nd) whereas a lower T N is described for R=Sm→Lu [30][31][32][33]. Additionally, some magnetic rareearths become long-range ordered, sometimes with different periodicity, as observed for HoNiO 3 [32].
This work describes the preparation of these novel, extremely distorted, nickel perovskites, and the structural investigation across the MI transition of the x=0.1 specimen, showing the evolution from the roomtemperature (RT) monoclinic and insulating phase to the high-temperature orthorhombic, metallic phase. High angular resolution SXRD is required since the metric of the monoclinic unit-cell at RT involves monoclinic β angles below 90.15°, implying a strong pseudo-orthorhombic character. The magnetic properties display a reduction of the Néel temperature, which is also consistent with the increment of the structural distortion observed by diffraction methods.

Experimental
Polycrystalline samples of Lu 1−x Sc x NiO 3 (x=0, 0.1, 0.2) were prepared under high-pressure conditions in a Kawai-type multianvil module (Max Voggenreiter GmbH). The x=0 perovskite, LuNiO 3 , was prepared for comparative purposes. The starting materials (0.5-x/2)Lu 2 O 3 , (x/2)Sc 2 O 3 , and Ni(OH) 2 in the stoichiometry ratio are thoroughly mixed with ∼30 wt.% KClO 4 , which serves as the oxidizing agent. The precursor powders were placed in a gold capsule, sealed and set in a cylindrical graphite heater. All these sample assembly was contained in a semi-sintered octahedron made of Ceramacast 584-OF. The pressure was generated by compressing the octahedron (edge length 14mm) with eight pieces of WC anvils with a truncated edge length of 8 mm. For each synthesis, the sample was subjected to heat treatment at 900°C for 20 min under 10 GPa. The temperature was quenched to room temperature before releasing pressure slowly. The resultant products were washed with water to dissolve KCl and then dried in air at 100°C for 2 h. Phase purity of the obtained Lu 1−x Sc x NiO 3 (x=0, 0.1, 0.2) polycrystalline samples was assessed by x-ray powder diffraction (XRD) at room temperature with CuKα radiation.
SXRD patterns were collected in the powder diffraction station of the MSPD beamline at the ALBA synchrotron, Barcelona (Spain), with 38 keV energy, λ = 0.3252 Å, and with the high angular resolution MAD set-up [34]. A selected sample Lu 0.9 Sc 0.1 NiO 3 was contained in a quartz capillary of 0.5 mm diameter. The acquisition temperatures were 25°C (RT) and 100, 150, 200, 250, 280, 300, 320 and 350°C. The refinement of the structures was performed by the Rietveld method. DC magnetic susceptibility was measured with a commercial Magnetic Property Measurements System (MPMS-III, Quantum Design) in the temperature range 2 K∼300 K under an external magnetic field of 0.5 T after field-cooled from RT. The peak marked by an asterisk is the main peak of KCl coming from KClO 4 , which would be washed away before characterization. Lattice parameters obtained from the Rietveld refinements from laboratory XRD patterns are illustrated in figure 1(b). With increasing x in the Lu 1−x Sc x NiO 3 series, a and c decrease, but b increases with a net decrease of unit-cell volume V, in line with the fact that Sc 3+ has a smaller ionic radius than Lu 3+ .

Results and discussion
The crystal structure of Lu 0.9 Sc 0.1 NiO 3 at RT, in the insulating region, was refined in the monoclinic P2 1 /n space group. Above T MI , the structure was defined in the orthorhombic Pbnm space group; the abrupt variation of the unit-cell parameters, particularly the monoclinic beta angle, described below, indicated T MI =320°C, slightly lower than that of LuNiO 3, of 326°C [10]. The SXRD patterns after the Rietveld refinement at RT and 350°C are displayed in figure 2. The plots at the remaining temperatures are gathered in figure S1 is available online at stacks.iop.org/MRX/7/126301/mmedia of the Supplementary Information. Thanks to the excellent crystallinity and the high angular resolution of the MSPD diffractometer, it was possible to resolve certain characteristic peak splittings in the monoclinic phase, as illustrated in the insets of figure 2, for the (−2 2 4) and (2 2 4) reflections, that merge into a single peak above T MI . Therefore, the crystal structure at 25°C and below the transition temperature is defined in the P2 1 /n symmetry, as proposed for RNiO 3 perovskites [7], with characteristic unit-cell parameters » » » a a b a and c a 2 , 2 2 , 0 0 0 where a 0 defines the unit-cell of the simple cubic perovskite. In this structure Ni1 and Ni2 are located at 2d and 2c sites, respectively, and O1, O2 and O3 oxygen atoms at 4e Wyckoff sites. Table 1(a) lists the main crystallographic parameters at RT. In the crystal structure there are small Ni1O 6 and large Ni2O 6 octahedra alternating along the three direction, as displayed in figure 1(c). This arrangement has been interpreted as a charge disproportionation effect [7][8][9][10][11]25]. It is remarkable the small monoclinic β angle at RT, of 90.15°, indicating a strong pseudo-orthorhombic character. This value compares with those reported in RNiO 3 (R=Ho, Y, Er and Lu) perovskites, with β angles ranging from 90.08°for R=Y, Ho to 90.16°for LuNiO 3 at RT [9].
Above T MI , in the metallic region, the crystal framework corresponds to the standard Pbnm orthorhombic superstructure of perovskite, with a single Ni atom at 4b sites and two O1 and O2 atoms at 4c and 8b Wyckoff positions, respectively (table 1(b)). This is the conventional description of the GdFeO 3 -type perovskite structure [1].
The variation of the structural parameters across the phase transition at T MI =320°C has been investigated from SXRD data ( figure 3). The a and b unit-cell parameters regularly increase in the measured temperature interval; a conspicuous contraction of c parameter is realized upon entering the metallic regime across the phase transition. Interestingly, while a decrease of b was described for large R perovskites (R=Pr, Nd, Sm) below T MI [27], an increment of the b lattice parameter occurs for RNiO 3 with smaller rare earths (R=Ho, Y, Er, Lu) [10].
It is interesting to compare the evolution of the interatomic Ni-O bonds between the monoclinic and the orthorhombic phases. Table 2 included the main Ni-O interatomic distances for the two types of NiO 6 octahedra in the P2 1 /n structure (at 25°C), and the single type of NiO 6 octahedron at the Pbnm s.g. (at 350°C). The average 〈Ni-O〉distances indicate that, in the monoclinic model, Ni1O 6 octahedron is significantly smaller Figure 3. Temperature variation of the unit-cell parameters and volume across the MI transition (T MI =320°C). Immediately above T MI the symmetry becomes orthorhombic, and the monoclinic angle abruptly falls to 90°. Table 1.  [35]. In the monoclinic phase, the BVS for Ni1 and Ni2 are 3.41+and 2.81+, respectively, significantly above and below the nominal value of 3+, (table 2), indicating the mentioned charge disproportionation, Ni 3+δ and Ni 3-δ . In average, the δ value is 0.30, as observed in other members for small rare-earth cations [9]. Figure 5(a) illustrates the thermal variation of the dc magnetic susceptibility χ(T) for these three samples. The antiferromagnetic (AFM) transition at T N is clearly manifested as a kink in χ(T). This singularity broadens up with increasing of the Sc content, probably due to the enhanced lattice distortion with Sc doping. It also Table 2. Main bond distances (Å) for monoclinic (at 25°C) and orthorhombic (at 350°C) Lu 0.9 Sc 0.1 NiO 3 . For the calculation of the bond valence sums (BVS) the following parameters were used: B=0.37, R 0 (Lu 3+ )=1.971, R 0 (Sc 3+ )=1.849, R 0 (Ni 2+ )= 1.654 [35].  shows that T N decreases linearly with x in Lu 1−x Sc x NiO 3 series; a decrease of the T N ordering temperature from 122 K (x=0) to 113 K (x=0.2) is observed, which is connected with a more distorted perovskite structure with reduced superexchange Ni-O-Ni angles. Former investigations in the RNiO 3 perovskite series described low-spin Ni(III) as ground state [32], also supported in further studies [33]. Moreover, Curie-Weiss fits unveil paramagnetic moments (sufficiently above the onset for magnetic ordering) suggesting a low-spin 3t 2g 6 e g 1 electronic configuration, S=1/2, as illustrated for YNiO 3 [36]. In complement, the magnetic structures investigated by neutron diffraction at low temperatures disclose ordered magnetic moments compatible with S=½ [8,37]. Figure 5(b) shows a complete RNiO 3 phase diagram with the novel T N and T MI points, corresponding to the smallest-sized R 3+ members of the series. Based on the classical diagram published by Torrance et al [5], showing a divergence between the metal-insulator transitions, T MI , and the antiferromagnetic ordering temperature, T N , for rare-earth sizes slightly smaller than Nd 3+ , we add a somewhat smaller T MI for Lu 0.9 Sc 0.1 NiO 3 than that observed for LuNiO 3 , as well as reduced T N 's for the Sc 0.1 and Sc 0.2 members, thus exhibiting the lowest T N of the full RNiO 3 series, given the strongest distortion exhibited by these perovskites.

Conclusions
We have demonstrated that an increased perovskite distortion beyond LuNiO 3 is possible in the series Lu 1−x Sc x NiO 3 . Very high pressure of 10 GPa is required for the stabilization of the novel members. The samples present MI transitions with concomitant structural changes, evolving from a monoclinic phase below T MI to an orthorhombic symmetry in the metallic high-T structure. A dramatic rearrangement of the unit-cell parameters is observed when approaching T MI =320°C for x=0.1. The monoclinic, low-T insulating phase contains two types of Ni octahedra, corresponding to a charge-disproportionation effect. The average charge disproportionation between Ni1 and Ni2 is about ±0.3, as observed in other RNiO 3 members with small rareearth cations. The magnetic properties show an antiferromagnetic ordering with T N decreasing as the Sc content increases, as corresponds to a more distorted perovskite structure.