A series of britholite compounds were synthesized by simultaneous introduction of trivalent La and Si ions into an apatite structure. The variations in the average structure, electronic band structure, and microstructural properties resulting from the introduction of cation pairs were analyzed as a function of their concentration. The effects of the structural variance and microstructural properties on the broad-band-emitting activator ions were studied by introducing Eu ions as activators. For the resulting compound, which had dual emission bands in the blue and yellow regions of the spectrum, the emission peak position and strength were dependent upon the concentration of La-Si pairs. By engineering the relative sizes of the two possible activator sites in the structure, 4f and 6h, through the introduction of a combination of trivalent La and a polyanion, the preferential site occupancy of the activator ions was favorably altered. Additionally, the activator ions responsible for the lower-Stokes-shifted blue component of the emission functioned as a sensitizer of the larger-Stokes-shifted yellow-emitting activators, and predominantly yellow-emitting phosphors were achieved. The feasibility of developing a white light-emitting solid-state device using the developed phosphor was also demonstrated.
Over the last few decades, manipulating the metal-insulator (MI) transition in perovskite oxides (ABO3) via an external control parameter has been attempted for practical purposes, but with limited success. The substitution of A-site cations is the most widely used technique to tune the MI transition. However, this method introduces unintended disorder, blurring the intrinsic properties. The present study reports the modulation of MI transitions in [10 nm-NdNiO3/t-LaNiO3/10 nm-NdNiO3/SrTiO3 (100)] trilayers (t = 5, 7, 10, and 20 nm) via the control of the LaNiO3 thickness. Upon an increase in the thickness of the LaNiO3 layer, the MI transition temperature undergoes a systematic decrease, demonstrating that bond disproportionation, the MI, and antiferromagnetic transitions are modulated by the LaNiO3 thickness. Because the bandwidth and the MI transition are determined by the Ni-O-Ni bond angle, this unexpected behavior suggests the transfer of the bond angle from the lower layer into the upper. The bond-angle transfer eventually induces a structural change of the orthorhombic structure of the middle LaNiO3 layer to match the structure of the bottom and the top NdNiO3, as evidenced by transmission electron microscopy. This engineering layer sequence opens a novel pathway to the manipulation of the key properties of oxide nickelates, such as the bond disproportionation, the MI transition, and unconventional antiferromagnetism with no impact of disorder.
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