Temperature-dependent vibrational spectroscopic and X-ray diffraction investigation of nanosized nickel chromite

Highlights

• The nanocrystalline nickel chromite was prepared by auto-combustion method.

• The nanocrystals were characterized by powder X-ray diffraction, vibrational spectroscopy and magnetic measurements.

• The expected structural phase transitions (cubic–tetragonal–orthorhombic) were studied form 350 K down to 4 K.

• The evolution of the Raman spectra and X-ray diffraction patterns confirmed the cubic-to-tetragonal distortion at ∼250 K.

• The tetragonal-to-orthorhombic transition was not confirmed in the nanocrystalline sample.

Abstract

The nanocrystalline nickel chromite (NiCr₂O₄) with particle size of ∼20 nm was prepared by auto-combustion method. The nanocrystals were characterized by powder X-ray diffraction, vibrational spectroscopy and magnetic measurements. The expected structural phase transitions (cubic–tetragonal–orthorhombic) were studied by methods of temperature-dependent X-ray powder diffraction and vibrational spectroscopy. The evolution of the Raman spectra and X-ray diffraction patterns collected from 350 K down to 4 K confirmed the cubic-to-tetragonal distortion at ∼250 K, whereas the tetragonal-to-orthorhombic transition was not confirmed in the nanocrystalline sample.

Introduction

The NiCr₂O₄ crystallizes in the normal spinel structure with the face-centered cubic space group (Fd-3m) and the atoms of NiCr₂O₄ occupy 8a (tetrahedral – T_d, Ni²⁺), 16d (octahedral – D_3d, Cr³⁺) and 32e (C_3v, O²⁻) Wyckoff sites. A sublattice of Ni²⁺ cations forms a diamond-type lattice with tetrahedral oxygen environment and sublattice of corner-shared Cr³⁺ surrounded by octahedral oxygen cages forms a pyrochlore-type structure.

At temperatures above 310 K, the crystal structure of the normal spinel belongs to the cubic system (Fd-3m) with the atomic sites contributing to F_2g + F_1u (Ni²⁺), A_2u + E_u + F_2u + 2F_1u (Cr³⁺) and A_1g + E_g + 2F_2g + F_1g + A_2u + E_u + F_2u + 2F_1u (O²⁻) modes of the theoretical vibrational representation. From these modes, the A_1g, F_2g (triply degenerate) and E_g (doubly degenerate) are active in Raman spectra while the four F_1u (triply degenerate) are IR active and one remaining F_1u (acoustic mode) cannot be observed in infrared spectra. The rest of modes are Raman inactive (F_1g) and infrared inactive (2A_2u, 2E_u, and 2F_2u).

The interaction of the spin, orbital and phonon subsystems can lead to the appearance of the Jahn–Teller distortion. In spinel chromites, the oxygen atoms form a weak crystal field resulting in low-spin configuration of d orbitals, hence distortion should be expected only in the case where the d⁵, d⁶, d⁸ and d⁹ cation of transition metal occupy the tetrahedral site A. This assumption is confirmed in the case of nickel (Ni²⁺, d⁸) [1], [2], [3], [4], [5] and copper (Cu²⁺, d⁹) [1], [5], [6]. The features of Jahn–Teller transition were studied in the systems of Ni_1−xCo_xCr₂O₄ and NiFe₂O₄–NiCr₂O₄–CuCr₂O₄ solid solutions by synchrotron X-ray powder diffraction [7], [8].

In the NiCr₂O₄, the lowering of the cubic O_h symmetry to the tetragonal D_4h symmetry (I4₁/amd space group), crystal structure described in [2], which occurs due to Jahn–Teller effect below room temperature, is connected with splitting of the triply degenerate modes F_1u (to A_2u and E_u) and F_2g (to B_2g and E_g), and of doubly degenerate mode E_g (to A_1g and B_1g). Additional E_g and 2E_u bands can appear arising from the silent modes (in cubic phase) F_1g and 2F_2u in the Raman and infrared spectra, respectively. The atoms of NiCr₂O₄ occupy 4a (D_4d, Ni²⁺), 8d (C_2h, Cr³⁺) and 16 h (C_s, O²⁻) Wyckoff sites in the tetragonal space group. The atomic sites contribute to B_1g + E_g + A_2u + E_u (Ni²⁺), B_1u + A_1u + 2B_2u + 2E_u + A_2u (Cr³⁺) and 2A_1g + B_2g + 2B_1g + 3E_g + A_2g + B_1u + A_1u + 2B_2u + 3E_u + 2A_u (O²⁻) modes of the theoretical vibrational representation. From these modes, the A_1g, B_1g, B_2g, and E_g (doubly degenerate) are active in Raman spectra while the A_2u and E_u (doubly degenerate) are IR active and the remaining modes are inactive in both spectra.

Additional symmetry lowering is connected with transition from the tetragonal D_4h symmetry (I4₁/amd space group) to the orthorhombic D_2h symmetry (Fddd space group), which results in the splitting of all originally degenerate modes. The four originally triple degenerate infrared-active F_1u modes of the cubic phase (4F_1u) should split in the orthorhombic phase into B_1u + B_2u + B_3u modes. The five Raman-active modes A_1g, E_g and 3F_2g are converted into the 3A_g, 3B_1g, 3B_2g and 3B_3g modes. The additional bands arising from the silent modes (in the tetragonal phase) A_2g and 2B_2u can be observed as B_1g and 2B_1u modes in Raman and infrared spectra, respectively. The atoms of NiCr₂O₄ occupy 8a (D₂, Ni²⁺), 16d (C_i, Cr³⁺) and 32 h (C₁, O²⁻) Wyckoff sites in the orthorhombic space group. These atomic sites contribute to B_1g + B_2g + B_3g + B_1u + B_2u + B_3u (Ni²⁺), 3A_1u + 2B_1u + 2B_2u + 2B_3u (Cr³⁺) and 3A_g + 3A_u + 3B_1g + 2B_2g + 3B_3g + 3B_1u + 3B_2u + 3B_3u (O²⁻) modes of the theoretical vibrational representation. From these modes, the A_g, B_1g, B_2g, and B_3g are active in Raman spectra while the B_1u, B_2u and B_3u are IR active and the remaining modes are inactive in both spectra.

The above described sequence of the structural transitions is directly related to unusual magneto-dielectric behavior of the NiCr₂O₄ phase [9], [10]. Below 60 K the system orders ferrimagnetically and at 30 K it undergoes an order to order magnetic phase transition [11]. Recently, an additional phase transition at 20 K has been observed by magnetocapacitance measurements and has been attributed to completion of ferrimagnetic ordering and the spin-driven structural distortion [10].

A recent study was motivated by the preparation of the nickel chromite nanoparticles, which symmetry at room temperature (tetragonal or cubic) is influenced by the method used for their preparation [12]. In this work, we focused on investigation of the temperature evolution of the crystal structures of nickel chromite nanoparticles with size of ∼20 nm by temperature-dependent vibrational spectroscopy and X-ray powder diffraction experiments.