Temperature-induced valence-state transition in double perovskite Ba2−xSrxTbIrO6

In this paper, a temperature-induced valence-state transition is studied in a narrow composition range <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>0.2</mn><mo>≤</mo><mi>x</mi><mo>≤</mo></mrow></math> 0.375 of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>Ba</mi><mrow><mn>2</mn><mo>-</mo><mi>x</mi></mrow></msub><msub><mi>Sr</mi><mi>x</mi></msub><msub><mi>TbIrO</mi><mn>6</mn></msub></mrow></math> by means of x-ray and neutron powder diffraction, resonant inelastic x-ray scattering, magnetic susceptibility, electrical resistivity, and specific heat measurements. The valence-state transition involves an electron transfer between Tb and Ir leading to the valence-state change between <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>5</mn><mo>+</mo></mrow></msup></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup></mrow></math> phases. This first-order transition has a dramatic effect on the lattice, transport properties, and the long-range magnetic order at low temperatures for both Tb and Ir ions. <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>Ir</mi></mrow><mrow><mn>5</mn><mo>+</mo></mrow></msup></math> ion has an electronic configuration of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>5</mn><msup><mi>d</mi><mn>4</mn></msup></mrow></math> (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>J</mi><mi>eff</mi></msub><mo>=</mo><mn>0</mn></mrow></math>), which is expected to be nonmagnetic. In contrast, <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>Ir</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup></math> ion with a configuration of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>5</mn><msup><mi>d</mi><mn>5</mn></msup><mrow><mo>(</mo></mrow><msub><mi>J</mi><mi>eff</mi></msub></mrow></math> = 1/2) favors a long-range magnetic order. For <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>x</mi><mo>=</mo><mn>0.1</mn></mrow></math> with <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>5</mn><mo>+</mo></mrow></msup></mrow></math> configuration to the lowest temperature (2 K) investigated in this paper, a spin-glass behavior is observed around 5 K indicating <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>Ir</mi></mrow><mrow><mn>5</mn><mo>+</mo></mrow></msup></math> (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>J</mi><mi>eff</mi></msub><mo>=</mo><mn>0</mn></mrow></math>) ions act as a spacer reducing the magnetic interactions between <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>Tb</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup></math> ions. For <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>x</mi><mo>=</mo><mn>0.5</mn></mrow></math> with <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup></mrow></math> configuration below the highest temperature 400 K of this paper, a long-range antiferromagnetic order at <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>T</mi><mi mathvariant="normal">N</mi></msub></math> = 40 K is observed highlighting the importance of <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>Ir</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup></math> (<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>J</mi><mi>eff</mi></msub></math> = 1/2) ions in promoting the long-range magnetic order of both Tb and Ir ions. For 0.2 <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>≤</mo><mi>x</mi><mo>≤</mo></mrow></math> 0.375, a temperature-induced valence-state transition from high-temperature <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>3</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>5</mn><mo>+</mo></mrow></msup></mrow></math> phase to low-temperature <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mrow><mi>Tb</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup><mo>/</mo><msup><mrow><mi>Ir</mi></mrow><mrow><mn>4</mn><mo>+</mo></mrow></msup></mrow></math> phase occurs in the temperature range 180 K <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>≤</mo><mi>T</mi><mo>≤</mo></mrow></math> 325 K and the transition temperature increases with <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi></math>. The compositional dependence demonstrates the ability to tune the the valence state for a critical region of <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi></math> that leads to a concurrent change in magnetism and structure. Furthermore, this tuning ability could be employed with suitable strain in thin films to act as a switch as the magnetism is manipulated.