Experimental Observation of Monoclinic Distortion in the Insulating

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Experimental Observation of Monoclinic Distortion in the Insulating Regime of SmNiO3 by Synchrotron X‑ray Diffraction Federico Serrano-Sań chez,† François Fauth,‡ Jose ́ Luis Martínez,† and Jose ́ Antonio Alonso*,† †

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Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Sor Juana Inés de la Cruz 3, E-28049, Madrid, Spain ‡ CELLS−ALBA Synchrotron, Cerdanyola del Valles, E-08290 Barcelona, Spain ABSTRACT: RNiO3 (R = rare-earth element) perovskite materials are well-known to exhibit characteristic metal−insulator transitions. The structural distortion increases as the R member becomes smaller along the series. For SmNiO3, a high-hydrostatic-pressure preparation procedure, yielding samples with much enhanced crystalline quality, combined with the extremely high angular resolution of synchrotron X-ray diffraction (XRD) allowed us to identify a monoclinic phase in the insulating regime (below the metal−insulator transition temperature (TMI) of 127 °C), defined in the space group P21/n. This monoclinic symmetry had not been demonstrated directly using nonresonant XRD or neutron diffraction. This has important repercussions on the electronic nature of this material since the monoclinic structure contains two inequivalent Ni positions, implying a charge disproportionation phenomenon. In the metallic regime (above TMI), the standard orthorhombic Pbnm structure is observed. Therefore, there is a coupled structural and electronic transition, as happens for the very small rare-earth compounds of the RNiO3 perovskite series. Across TMI there is a dramatic rearrangement of the lattice parameters, degree of tilting, and distortion of the NiO6 octahedra, showing the convergence of the Ni−O bond lengths upon entering the metallic phase. Brown’s valence analysis of the different elements agrees with other reported values in the literature, matching with bond and charge disproportionation models. By magnetization measurements a Néel temperature (TN) corresponding to the antiferromagnetic ordering of the Ni moments is identified at TN= 220 K, whereas Sm moments experience long-range ordering below 36 K.

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INTRODUCTION Correlated oxides with transition metal cations display remarkable physical phenomena such as spin ordering, highTc superconductivity, colossal magnetoresistance, and metal− insulator (MI) transitions.1−3 Among them, RNiO3 perovskites (R = Y or lanthanide), nominally containing Ni3+ ions, have been thoroughly studied because of their characteristic MI transitions,4−7 which happen as a function of temperature or the rare-earth ionic size. In fact, the perovskite RNiO3 family to date represents the most classic case of electronic state evolution from Pauli metal to magnetic Mott insulator. These MI transitions have been associated with strong electron correlation effects, and thus, RNiO3 oxides are considered as remarkable systems to study fundamental problems of condensed matter. Their properties make them outstanding candidates for applications as multiferroic materials,8 superlattices,9,10 fuel cells,11 memory devices,12 and bioelectronic interfaces.13 The covalent character of the Ni−O bonds affects the band width in these compounds, which decreases as R is varied from La to Lu. This is consequence of the lanthanide contraction, as the R ionic size determines the perovskite tolerance factor, and thus, the smaller the cation, the more distorted is the perovskite structure.6,14 The first member of the series, © XXXX American Chemical Society

LaNiO3, presents a rhombohedral R3̅c structure and metallic behavior over a wide temperature range.15,16 The remaining rare-earth derivatives (R = Pr−Lu) display a high-temperature orthorhombic structure and a sharp MI transition at MI transition temperatures (TMI) in the range from 130 to 600 K depending on the nature of the rare-earth cation.14,17−19 High-resolution structural studies revealed that a structural transition is coupled to the electronic transition for the smaller rare-earth compounds (R = Y, Dy−Lu), from a lowtemperature monoclinic insulating phase, concomitant with a Ni charge disproportionation effect, to the high-temperature orthorhombic metallic phase, which has been reported and analyzed in numerous works.20−25 By contrast, for the larger rare-earth derivatives (R = Pr−Gd), the orthorhombic symmetry has been described well below TMI19,26 because of the minor structural distortions associated with the transition. However, despite the intangibility of the monoclinic distortion, it has been described for some cases, such as the Pr derivative from high-resolution neutron powder diffraction (NPD) data27 and the Nd perovskite using resonant X-ray scattering.28 Moreover, Ni charge disproportionation has been observed by Received: July 7, 2019

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DOI: 10.1021/acs.inorgchem.9b02013 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

gold capsule with a diameter of 5 mm, which was put inside a cylindrical graphite heater. The high-pressure reaction was carried out in a Rockland Research piston−cylinder press under 3.5 GPa at 900 °C for 20 min. The reaction was quenched, after which the pressure was released. In order to remove KCl leftovers from the decomposition of KClO4 and other impurity phases as traces of R2O3 and NiO, the product powder was washed with dilute HNO3 solution. Finally, the sample was dried at 150 °C for 1 h in air. The phase purity was determined by X-ray diffraction (XRD) on a Bruker AXS D8 diffractometer (40 kV, 30 mA) with Cu Kα radiation (λ = 1.5418 Å) run by DIFFRACTPLUS software in the Bragg− Brentano reflection geometry. The Ni oxidation state, determined by iodometry, was 2.97(5)+, indicating a trivalent oxidation state within the standard deviations. SXRD experiments were carried out in transmission mode at the BL04-MSPD beamline of the ALBA synchrotron (Barcelona, Spain) using the highest angular resolution mode as provided by the MAD setup.42 Fine powders of SmNiO3 material were sealed in 0.7 mm diameter quartz capillaries that were rotated during the acquisition time to increase powder averaging. The beam energy was 28 keV (λ = 0.4427 Å), selected to optimize absorption. Temperature-dependent SXRD patterns were collected at 25 °C (RT) for both S1 and S2 and then at 70, 110, 122, 135, 150, 200, and 250 °C for S2. The Rietveld refinement of the SXRD data was performed using the FullProf program. A pseudo-Voigt function was employed to define the diffraction peak shape; the background was refined as a linear interpolation between points devoid of reflections. The complete analysis included refinement of the scale factor, zero-point shift, asymmetry parameters, occupancy factors, atomic positions, and isotropic displacement factors. The extremely good data quality (combination of high flux and angular resolution) of the synchrotron radiation even allowed us to include the refinement of the atomic positions and occupancy factors of oxygen atoms, which were obtained with tolerable accuracy. No regions were excluded from the refinement. Differential scanning calorimetry (DSC) measurements were performed using a Mettler TA3000 system equipped with a DSC30 unit over the temperature range from 50 to 240 °C. The heating and cooling rates were 10 °C min−1, and about 70 mg of sample was used.

high-resolution X-ray absorption spectroscopy in the insulating state of the whole RNiO3 family, providing further evidence that the monoclinic distortion is concomitant with the insulating state.29 The physical nature of this transition has been a characteristic target of highly specialized techniques, such as resonant inelastic X-ray scattering,30,31 ultrafast optical spectroscopy,32,33 total neutron scattering,34 and broad-band dielectric spectroscopy.34 Moreover, these compounds exhibit antiferromagnetic ordering at low temperature, for which the Néel temperature (TN) is equal to TMI for larger R cations (R = Pr, Nd) and lower than TMI for R = Sm−Lu.35−38 Since the first reports of the MI transition,4−6 an exhaustive description of the electronic and structural properties of these materials has been given experimentally and theoretically. The overlap of Ni 3d and O 2p orbitals dominates the electronic transport across the crystal structure, characterizing the insulator and the metallic behavior. The first diffraction studies of the structural monoclinic distortion in the insulating regime20−22 attributed the electron localization to partial charge disproportionation of Ni3+ (metallic regime) into Ni3+δ and Ni3−δ (insulating regime). It has also been associated with a breathing distortion of the octahedra due to the asymmetric distribution of two distinct Ni atoms (Ni1 and Ni2) at the center of the octahedra with different average Ni−O bond lengths.25,39,40 Currently, the more common picture of the structural distortion is that of a bond disproportionation. Ni atoms are described in a d8 electronic configuration, and a positive charge is distributed among the oxygen positions, given the importance of Ni 3d and O 2p hybridization. Recent spectroscopic studies suggest a configuration of 3d8L1 → 3d8+3d8L2 in the disproportionated state, in which the 2p−2p insulating band gap would be within the hybridized oxygen band.34,39,41 However, a comprehensive model describing the physical properties of these systems is still needed. The true crystallographic nature of SmNiO3 perovskite has been elusive until now; this paper presents the first experimental report of the monoclinic crystallographic structure of SmNiO3 polycrystalline oxide prepared under high- pressure. A novel preparation procedure combined with the extremely high angular resolution of synchrotron X-ray diffraction (SXRD) allowed us to identify a monoclinic phase in the insulating regime (below TMI = 127 °C), defined in the space group P21/n. This has important repercussions on the electronic nature of this material since the monoclinic structure contains two inequivalent Ni positions, suggesting a charge disproportionation phenomenon. This is the first evidence of charge disproportionation in SmNiO3 from diffraction data. The temperature evolution of the crystal structure across TMI (127 °C) is described, involving an important structural rearrangement as the material enters the metallic orthorhombic phase. A magnetic susceptibility study confirmed the antiferromagnetic ordering of Ni moments below 220 K.

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RESULTS AND DISCUSSION Preliminary Characterization: SXRD and DSC Measurements. As the most significant difference between the two SmNiO3 samples, we recall that S1 was obtained by annealing precursor powders under oxygen at a pressure of 200 bar (0.2 GPa) whereas S2 was synthesized under a high hydrostatic pressure of 3.5 GPa in the presence of KClO4. Figure 1 shows a selected region of the SXRD patterns of the two samples collected at RT. They both correspond to single perovskite phases. Whereas S1 presents enlarged peaks, S2 displays much sharper reflections, indicating superior crystallinity. This can be appreciated in the reflection triplet in the 9.2−9.7° range, which is perfectly resolved for S2. The positions of the reflections indicate close unit-cell parameters for the two samples. S1 is comparable to the sample that was previously analyzed in ref 21 using SXRD, for which an orthorhombic symmetry was assigned at RT. Also, the same synthetic protocol under a moderate O2 pressure was used to prepare the samples where Lacorre et al.4,5 first described the MI transition in this family of materials. However, the present SXRD pattern of the S2 sample, obtained under high hydrostatic pressure, clearly enables visualization of the characteristic reflection splitting that suggests a monoclinic symmetry. Therefore, a complete temperature-dependent study was carried out only for the S2 specimen in order to analyze the structural evolution across the metal−insulator transition. This

EXPERIMENTAL SECTION

Two samples of SmNiO3 were prepared by different protocols. The first one, S1, was obtained by a citrate technique by dissolving Sm2O3 and Ni(NO3)2·6H2O in a 10% citric acid solution, forming a resin by gentle heating, and decomposing the organic matrix at 600 °C to yield a reactive precursor powder. This precursor was finally annealed at an O2 pressure of 200 bar for 12 h at 900 °C in a Morris Research furnace. The second sample, S2, was prepared under high hydrostatic pressure from Ni(OH)2 and Sm2O3 intimately mixed with 30% KClO4. A 1.0 g sample of the precursor mixture was encapsulated in a B


Article

Inorganic Chemistry

Figure 1. Comparison of the SXRD patterns of samples S1 (annealed under O2 at 0.2 GPa) and S2 (synthesized at a hydrostatic pressure of 3.5 GPa), illustrating the much sharper reflections exhibited by S2.

transition was previously characterized by DSC measurements; Figure 2 displays the DSC curves for the heating and cooling

Figure 3. Rietveld plots from the SXRD data for S2 at (a) RT and (b) 250 °C, corresponding to structural refinements in the monoclinic space group P21/n and orthorhombic space group Pbnm, respectively. The insets show the characteristic monoclinic splitting betweenthe (2̅24) and (224) reflections, which vanishes above TMI. The second series of Bragg reflections correspond to a minor impurity of NiO.

the diffraction patterns obtained at the MSPD beamline allows the observation of the characteristic monoclinic peak splitting below TMI, as displayed by the distinctive splitting of the (2̅24) and (224) reflections (Figure 3a inset), which has not been reported previously. Consequently, it was possible to refine the insulating phase (below TMI) as monoclinic, and its structural parameters are described here along with a temperaturedependent structural study near TMI. At RT and below TMI, the crystal structure was refined in the previously reported P21/n structure of RNiO3 perovskites,21 with typical lattice parameters a ≈ 2 a0, b ≈ 2 a0 , and c ≈ 2a0, where a0 is the edge of the cubic aristotype. This model contains two different Ni atoms, Ni1 and Ni2, located at 2d and 2c sites, respectively, and three O atoms, O1, O2, and O3, located at 4e general crystallographic positions. The structural parameters at room temperature are included in Table 1. A minor NiO impurity was detected and included as a secondary phase in the refinement. Alternating small Ni1O6 and large Ni2O6 octahedra are observed in the structural representation (Figure 4a), with the relevant Ni−O bond distances displayed. The monoclinic β angle at RT is as low as 90.04°, representing

Figure 2. DSC curves of SmNiO3 (S2 sample) for heating, showing an endothermic peak, and cooling (two cycles).

runs. An endothermic peak at 127 °C (400 K) was observed during the heating run, corresponding to the MI transition; the cooling run presents an exothermic peak at 125 °C, showing a small hysteretic effect ascribed to the first-order nature of the electronic transition. These results are in agreement with previous thermal analysis data for SmNiO3,6 which reported TMI = 403 K; the slightly lower TMI in the present case could be related to a superior degree of oxygenation achieved under high hydrostatic pressure. Structural Analysis from SXRD Data. The refinement of the crystal structure of the S2 specimen from SXRD data below TMI was performed in the monoclinic space group P21/n, while above TMI the orthorhombic space group Pbnm was considered. Figure 3 shows the diffraction patterns at room temperature and at 250 °C. They display excellent crystallinity with extremely narrow peaks (half-maximum width of typically 0.01° at 2θ = 12°), which is essential to discern the structural distortion accompanying the transition. The high resolution of C


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Inorganic Chemistry Table 1. Structural Parameters of SmNiO3 in Space Group P21/n at Room Temperature As Determined by SXRD

Table 2. Structural Parameters of SmNiO3 in Space Group Pbnm at 250 °C As Determined by SXRD

means of the typical a−a−c+ tilting of the octahedra, which is a consequence of the small ionic size of the Sm3+ ion. It is worth commenting that although the tilts and metrics are virtually the same in the two structures, there are some features in the patterns that afford the distinction between the orthorhombic and monoclinic symmetries. In particular, the appearance of a monoclinic distortion, coming from the splitting of the Ni sites into two inequivalent positions with large and small octahedra, implies the appearance of certain reflections (associated with the monoclinic symmetry) that can be conspicuously distinguished by high-resolution SXRD, like the (2̅24) and (224) couples (Figure 3). The thermal evolution of the structural parameters has been studied across the phase transition (Figure 5a). The a unit-cell parameter steadily increases for the whole temperature range, while there is a sharp decrease in b and c across the phase transition upon entering the metallic regime, concomitant with the collapse of the individual Ni1−O2 and Ni2−O3 bonds to two similar Ni−O2 and Ni−O2′ bonds of 1.962 and 1952 Å at 150 °C. However, these contractions, Δb ≈ −0.005 Å and Δc ≈ −0.003 Å, are smaller than those found for other RNiO3 compounds, which is in agreement with a less pronounced monoclinic distortion. These results are in agreement with previously reported work describing a reduction in b across the phase transition for large-R perovskites (R = Pr, Nd, Sm) that were described as orthorhombic phases below TMI. 19 Conversely, an increase in the b lattice parameter has been reported for smaller rare earths in RNiO3 (R = Ho, Y, Er, Lu).24 The variations of the unit-cell volume and the monoclinic β angle are plotted in Figure 5b. The calculated superimposed volume contraction associated with the vanishing of the charge disproportionation is ΔV/V = 0.18%, which is slightly larger than that previously reported (0.15%).44 Besides, in spite of a small deviation of the monoclinic phase from 90°, the abrupt reduction of the β angle across the transition upon entering the metallic state is similar to that reported for other Ni perovskites.24 The thermal evolution of the Ni−O bond distances has been plotted in order to analyze the symmetry breaking of the oxygen positions. Figure 6 shows the convergence of the different Ni−O bonds at the transition temperature, concomitant with the vanishing of the charge disproportionation. It is necessary to remark that the bond lengths are affected by relatively large errors due to the weak contribution of the scattering of the oxygen atoms to the diffraction peaks,

Figure 4. Two views of the crystal structure of SmNiO3: (a) in the monoclinic phase below TMI, showing large Ni2O6 and small Ni1O6 octahedra, and (b) in the orthorhombic phase in the metallic state above TMI, containing a single type of NiO6 octahedra.

the extremely subtle distortion of the monoclinic phase, which highlights the high resolution of the diffraction experiment. The refinement of the metallic phase was performed in the orthorhombic model, defined in the Pbnm space group, with a unique Ni atom at the 4b positions and two atoms, O1 and O2, at 4c and 8d Wyckoff sites, respectively. The structural parameters at 250 °C are included in Table 2. As observed in the Figure 1b inset, the structural rearrangement suppresses the splitting of the peaks in the diffraction pattern. Both the insulating and metallic phases optimize the Sm−O distances by D


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Inorganic Chemistry

Ni2−O and Ni1−O bonds of the P21/n phase are much more similar than in the other compounds, except for those described in the insulating phase of PrNiO3.27 Nevertheless, in Figure 6 the convergence of the six Ni−O bonds (monoclinic phase) into three different Ni−O bonds (orthorhombic phase) is clearly observed. As a consequence, the average length of Ni−O2 slightly decreases upon entering the phase transition while that of Ni−O3 increases, reaching an almost isotropic coordination environment of the tilted octahedra. The metallic phase displays Ni−O2 and Ni−O2′ lengths of 1.957−1.959 Å at 70 °C above TMI, exhibiting more regular octahedra than those described for R = Ho, Y, Er, and Lu, in agreement with previously reported data.24 In the orthorhombic phase at 150 °C, somewhat above TMI, the structure has undergone a notable distortion. The tilt angles of the octahedra have increased with respect to the insulating phase by ∼1°, which is coupled to the contraction of Ni−O bonds. This increased tilting, along with a similar average Ni−O2/Ni−O3 lengths in the monoclinic phase (1.958 Å at 122 °C) and the orthorhombic structure (1.959 Å at 150 °C) accounts for the decrease in the b lattice parameter. The octahedral distortion parameter, defined as Δd = 1 /6∑6n=1[(dn − ⟨d⟩)/⟨d⟩]2, describes how different the Ni−O bond distances are for each coordination unit. The values obtained for the insulating phase of SmNiO3 are 7 × 10−6 for Ni1 and 1.9 × 10−5 for Ni2 at room temperature. This is related to the fact that the small octahedra Ni1O6 correspond to Ni3+δ in the insulating, charge-disproportionated state, with a nominal electronic configuration t2g6eg1−δ, which tends to decrease the Jahn−Teller effect, yielding a more regular bond distribution than observed in the Ni2O6 octahedron, corresponding to a more distorted configuration. It is also remarkable that the distortion observed in SmNiO3 (the average Δd is 1.3 × 10−5) is much smaller than those observed for small-R perovskites. For instance, values of Δd as high as 2.57 × 10−4 (R = Ho) and 2.67 × 10−4 (R = Er) are observed immediately before the transition. In all of them, Δd increases up to the MI transition and then decreases at higher temperatures. Preceding works have also treated the charge localization below TMI as a consequence of a bond disproportionation effect in a model that establishes a positive charge at the oxygen position that is different for the Ni1 and Ni2 octahedra. It is possible to relate the charge disproportionation to the bond length and valence of the central atom using Brown’s ÅÄ r − r ÑÉ bond-valence model, in which expÅÅÅÅ 0 B i ÑÑÑÑ is proportional to the Ç Ö valence state of the central atom. From the bond distances in Table 3 at RT, the calculated values are 3.22+ and 2.60+ for Ni1 and Ni2 respectively. These values match perfectly with those given in the literature.20,21,24,27 The average value is still below the expected stoichiometric charge, which we would expect for a balanced compound. This could indicate the

Figure 5. Thermal evolution of the unit-cell parameters and unit-cell volume across the metal insulator transition (TMI = 127 °C). The monoclinic angle falls to 90° upon entering the orthorhombic metallic phase. The errors are smaller than the size of the symbols.

Table 3. Bond Distances, Distortion Parameter Values, and Brown’s Valence in SmNiO3 at RT

Figure 6. Temperature variation of the Ni−O distances across the MI transition: the six different bond lengths of the large and small octahedra in the monoclinic structure collapse into the Ni−O distances of the single NiO6 octahedron in the orthorhombic phase.

yielding significant standard deviations in the oxygen position parameters (see Tables 1 and 2). Moreover, in the SmNiO3 case, the charge disproportionation is not as noticeable as for the end members of the series, as the average lengths of the E


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Inorganic Chemistry presence of positive charge in the oxygen position, as has been suggested in bond disproportionation models. The calculated valence state of Ni above the phase transition is 2.92+ at 250 °C, which matches the expected valence of 3 within the experimental error. The same variation at the phase transition has been described for other RNiO3 with smaller R atoms, for which coupled structural and electronic transitions exist. This new description of the monoclinic phase in SmNiO3 confirms that the charge disproportionation phenomenon in the insulating regime is a general trend for the RNiO3 perovskites. NPD studies of PrNiO327 showed that the difference between the average Ni−O distances in the two Ni sites [0.052(2) Å] is almost 2 times smaller than the value reported for LuNiO3 [0.084(4) Å].24 This result suggests a progressive decrease in δ as the itinerant limit is approached and gives further support to the charge disproportionation mechanism as an origin of the metal−insulator transition in the whole RNiO3 family. The argument that “the charge modulation of Ni1 and Ni2 sites is independent of the cationic size” was found to be true for the smaller members of the series (Ho to Lu)24 and must be reconsidered from those findings for the large rare-earth analogues. In the case of SmNiO3 we find an intermediate situation between the Lu and Nd cases, since the difference between the average Ni−O distances for large and small octahedra at RT is 0.078 Å, which confirms the trend marked by the end case of PrNiO3. In the same manner, this finding regarding Sm also suggests the possibility that the intermediate-sized Eu and Gd nickel perovskite charge disproportionation could be unveiled by SXRD if the samples are prepared in a similar way in complement with the suitable high-angular-resolution diffraction techniques. Magnetic Properties. The top panel of Figure 7 displays the thermal evolution of the magnetic susceptibility of SmNiO3 (sample S2). It is dominated at low temperatures by the paramagnetic signal of the Sm3+ ion. The inset highlights a conspicuous inflection corresponding to the antiferromagnetic ordering of the Ni3+ spins at TN = 220 K. This value is in agreement with the value of 230 K given elsewhere,43 although the observed reduction may be related to the superior oxygenation degree of this sample prepared under higherpressure conditions. A similar effect was also described for the low-temperature shift of the TMI transition (see DSC Measurements). Initial studies of this family of compounds reported a low-spin ground state for Ni(III),44 and the rest of the literature agrees with this nominal picture as well.45 Not only that, but magnetic measurements found paramagnetic moments (well above the ordering temperature) compatible with this low-spin configuration 3t2g6eg1 (S = 1/2), for instance, for YNiO3.46 Also, the study of the magnetic structure at low temperatures by neutron diffraction found ordered magnetic moments compatible with S = 1/2. The low-temperature susceptibility peak with a cusp at 36 K should correspond to the long-range ordering of Sm 3+ magnetic moments. The neutron-absorbing character of natural Sm3+ has to date prevented the determination of the magnetic structure of this material; a study of an isotopically enriched 154Sm sample was carried out in an low-temperature orthorhombic Pnma symmetry.26 The bottom panel of Figure 7 shows the M versus H plots; the linear response at 300 K is characteristic of the paramagnetic state, whereas at T = 5 K a certain remnant magnetization with a magnetic hysteresis is observed, the origin of which is presently not very clear. It may

Figure 7. (a) Magnetic susceptibility vs temperature and (b) magnetization vs field plots for SmNiO3 (sample S2).

correspond to a canting of the Ni3+ or Sm3+ magnetic moments in the magnetic structure of this perovskite at low temperatures. The small value of the remnant magnetization, 0.002 μB/f.u., seems to suggest that it is likely related to Ni ions.

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CONCLUSIONS The structural phase transition of the SmNiO3 perovskite from the insulating phase with monoclinic space group P21/n below TMI = 127 °C to the metallic phase with the orthorhombic structure has been determined and analyzed for the first time using high-resolution synchrotron X-ray diffraction. A temperature-dependent investigation showed abrupt changes in the lattice parameters and Ni−O bond lengths across the phase transition, as previously reported for other RNiO3 perovskites with smaller R3+ ions. The evolution of the structure agrees with charge and bond disproportionation models and further settles that the charge modulation of the Ni1 and Ni2 sites depends on the cation size. This work confirms the previous hypothesis of monoclinic distortion in larger rare-earth perovskites concomitant with the MI transition and is an undeniable confirmation of charge disproportionation in SmNiO3, which was already accepted via sufficient indirect evidence. F


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Inorganic Chemistry

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(14) Medarde, M. L. Structural, Magnetic and Electronic Properties of RNiO3 Perovskites (R = Rare Earth). J. Phys.: Condens. Matter 1997, 9 (8), 1679. (15) Sreedhar, K.; Honig, J. M.; Darwin, M.; McElfresh, M.; Shand, P. M.; Xu, J.; Crooker, B. C.; Spalek, J. Electronic Properties of the Metallic Perovskite LaNiO3: Correlated Behavior of 3d Electrons. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46 (10), 6382−6386. (16) Aguadero, A.; Alonso, J. A.; Martínez-Lope, M. J.; FernándezDíaz, M. T.; Escudero, M. J.; Daza, L. In Situ High Temperature Neutron Powder Diffraction Study of Oxygen-Rich La2NiO4+δ in Air: Correlation with the Electrical Behaviour. J. Mater. Chem. 2006, 16 (33), 3402−3408. ́ (17) Alonso, J. A.; MartInez-Lope, M. J.; Rasines, I. Preparation, Crystal Structure, and Metal-to-Insulator Transition of EuNiO3. J. Solid State Chem. 1995, 120 (1), 170−174. (18) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; Martínez, J. L.; Demazeau, G.; Largeteau, A.; García-Muñoz, J. L.; Muñoz, A.; Fernández-Díaz, M. T. High-Pressure Preparation, Crystal Structure, Magnetic Properties, and Phase Transitions in GdNiO3 and DyNiO3 Perovskites. Chem. Mater. 1999, 11 (9), 2463−2469. (19) Garcia-Munoz, J. L.; Rodriguez-Carvajal, J.; Lacorre, P.; Torrance, J. B. Neutron-Diffraction Study of RNiO3 (R = La, Pr, Nd, Sm): Electronically Induced Structural Changes across the Metal−Insulator Transition. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46 (8), 4414. (20) Alonso, J. A.; García-Muñoz, J. L.; Fernández-Díaz, M. T.; Aranda, M. A. G.; Martínez-Lope, M. J.; Casais, M. T. Charge Disproportionation in RNiO3 Perovskites: Simultaneous Metal− Insulator and Structural Transition in YNiO3. Phys. Rev. Lett. 1999, 82, 3871−3874. (21) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; Aranda, M. A. G.; Fernández-Díaz, M. T. Metal−Insulator Transitions, Structural and Microstructural Evolution of RNiO3 (R = Sm, Eu, Gd, Dy, Ho, Y) Perovskites: Evidence for Room-Temperature Charge Disproportionation in Monoclinic HoNiO3 and YNiO3. J. Am. Chem. Soc. 1999, 121 (20), 4754−4762. (22) García-Muñoz, J. L.; Alonso, J. A.; Martínez-Lope, M. J.; Fernández-Díaz, M. T.; Casais, M. T. Room-Temperature Monoclinic Distortion Due to Charge Disproportionation in RNiO3 Perovskites with Small Rare-Earth Cations (R = Ho, Y, Er, Tm, Yb, and Lu): A Neutron Diffraction Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61 (3), 1756−1763. (23) Fernández-Díaz, M. T.; Alonso, J. A.; Martínez-Lope, M.; Casais, M. T.; García-Muñoz, J. L.; Aranda, M. A. G. Charge Disproportionation in RNiO3 Perovskites. Phys. B 2000, 276−278, 218−221. (24) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; GarcíaMuñoz, J. L.; Fernández-Díaz, M. T.; Aranda, M. A. G. HighTemperature Structural Evolution of RNiO3 (R = Ho, Y, Er, Lu) Perovskites: Charge Disproportionation and Electronic Localization. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64 (9), 094102. (25) Alonso, J. A.; Martínez-Lope, M. J.; Presniakov, I. A.; Sobolev, A. V.; Rusakov, V. S.; Gapochka, A. M.; Demazeau, G.; FernándezDíaz, M. T. Charge Disproportionation in RNiO3 (R = Tm, Yb) Perovskites Observed in Situ by Neutron Diffraction and 57Fe Probe Mössbauer Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87 (18), 184111. (26) Amboage, M.; Hanfland, M.; Alonso, J. A.; Martínez-Lope, M. J. High Pressure Structural Study of SmNiO3. J. Phys.: Condens. Matter 2005, 17 (11), S783−S788. (27) Medarde, M.; Fernández-Díaz, M. T.; Lacorre, P. Long-Range Charge Order in the Low-Temperature Insulating Phase of PrNiO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78 (21), 212101. (28) Staub, U.; Meijer, G. I.; Fauth, F.; Allenspach, R.; Bednorz, J. G.; Karpinski, J.; Kazakov, S. M.; Paolasini, L.; d’Acapito, F. Direct Observation of Charge Order in an Epitaxial NdNiO3 Film. Phys. Rev. Lett. 2002, 88 (12), 126402. (29) Medarde, M.; Dallera, C.; Grioni, M.; Delley, B.; Vernay, F.; Mesot, J.; Sikora, M.; Alonso, J. A.; Martínez-Lope, M. J. Charge

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Corresponding Author

*E-mail: [email protected]. ORCID

François Fauth: 0000-0001-9465-3106 José Antonio Alonso: 0000-0001-5329-1225 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Spanish Ministry of Science, Innovation and Universities through Grants MAT2017-84496R, MAT2014-52405-C2-2-R, and MAT2015-66888-C3-3-R cofinanced by FEDER. The authors express their gratitude to the ALBA technical staff for making the facilities available for synchrotron X-ray diffraction experiment number 2017072261.

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Experimental Observation of Monoclinic Distortion in the Insulating

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