Effect of nitrogen substitution on the structural and magnetic ordering transitions of NiCr2O4. Issue 113 (28th November 2016)

• Record Type: Journal Article
• Authors: Liu, Xin; Yin, Nan; Thomas, Tiju; Yang, Minghui; Wang, Junhu; Shi, Quan
• Is Part Of: RSC advances. Volume 6:Issue 113(2016)
• Page Start: 112140
• Page End: 112147
• Publication Date: 2016-11-28
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c6ra22773b
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : The nitrogen (N) doped spinel NiCr2 O4 has been synthesized at 773 K (N500) and 873 K (N600) by ammonolysis of NiCr2 O4 powders to study the effect of anion doping on its structural and magnetic properties. Abstract : The nitrogen (N) doped spinel NiCr2 O4 has been synthesized at 773 K (N500) and 873 K (N600) by ammonolysis of NiCr2 O4 powders to study the effect of anion doping on its structural and magnetic properties. The N contents are determined by thermogravimetric oxidation, yielding a composition that can be described as NiCr2 O3.68 N0.21 (N500) and NiCr2 O3.55 N0.30 (N600). X-ray photoelectron spectroscopic studies suggest that N 3− species partly substitute the oxygen in the lattice and oxygen vacancies exist in the N doped samples. There is evidence that in the nitrided sample, the Cr ion is most likely in a mixed oxidation state. As the N content increases, the structure at room temperature changes from tetragonal to the cubic phase; N500 is only partially tetragonal; N600 is completely cubic. Such structural change is the consequence of the depression of the cooperative Jahn–Teller effect of Ni 2+ in the tetrahedral A site caused by the presence of N 3− . Combined heat capacity and temperature dependent magnetic susceptibility measurements give clear evidence of the magnetic and structural transitions in the N doped NiCr2 O4 . The Jahn–Teller transition temperatures decrease with increasing N content; this is likely due to increased covalency and hence enhanced contribution of the angular momentum and the spin–orbit coupling to local chemical bonding around Ni 2+ . Antiferromagnetic transitions are observed at T S = 23 K and 22 K for N500 and N600, respectively. Hence there is indeed a lowering of transition when compared to pure NiCr2 O4 (28 K). The magnetic loops at different temperatures confirm that the material behaves as a paramagnet over a wide range of temperatures T ∼ 80–350 K. The material also exhibits a canted ferrimagnetic structural transition between 30 and 70 K. We also report evidence for increased frustration and lowered correlation length in N doped compounds compared to the parent NiCr2 O4 . The present study on N − doping effects on the structure and magnetic properties of this NiCr2 O4 is expected to be useful for tailoring the ferric phase transitions through anion substitutions.