Antithermal Quenching Upconversion Luminescence via Suppressed Multiphonon Relaxation in Positive/Negative Thermal Expansion Core/Shell NaYF4:Yb/Ho@ScF3 Nanoparticles

Thermal quenching (TQ) has been naturally entangling with luminescence since its discovery, and lattice vibration, which is characterized as multiphonon relaxation (MPR), plays a critical role. Considering that MPR may be suppressed under exterior pressure, we have designed a core/shell upconversion luminescence (UCL) system of α-NaYF4:Yb/Ln@ScF3 (Ln = Ho, Er, and Tm) with positive/negative thermal expansion behavior so that positive thermal expansion of the core will be restrained by negative thermal expansion of the shell when heated. This imposed pressure on the crystal lattice of the core suppresses MPR, reduces the amount of energy depleted by TQ, and eventually saves more energy for luminescing, so that anti-TQ or even thermally enhanced UCL is obtained.

Ultrasonic wave propagation in ternary intermetallic CeCuGe compound

The elastic and ultrasonic properties of the hexagonal intermetallic CeCuGe ternary compound have been evaluated along unique axis at room temperature. Higher order elastic constants have been calculated for CeCuGe compound using Lennard-Jones interaction potential model. The ultrasonic velocity, Debye average velocity, thermal relaxation time and acoustic coupling constant are evaluated along unique axis of the crystal and the temperature dependent ultrasonic velocities are evaluated along different angles with unique axis of the crystal. Finally temperature dependent ultrasonic attenuation is evaluated to extract important information about the material.

The elastic behaviour of orthorhombic sulphur under pressure

Orthorhombic, α -sulphur comprises S 8 rings strongly bound by covalent, intramolecular forces but held together by much weaker intermolecular bonds. To examine the vibrational anharmonicity of the long-wavelength acoustic modes in a molecular crystal of this type, the effects of hydrostatic and uniaxial pressures upon the velocities of ultrasonic modes propagated in a single crystal of orthorhombic sulphur have been measured. From the experimental results at room temperature 19 of 20 third-order elastic constants and also the hydrostatic pressure derivatives of the 9 second-order elastic constants have been obtained. The elastic stiffnesses and their hydrostatic pressure derivatives have also been calculated from an intermolecular potential of the 6-exp variety. Good agreement between the results obtained from this lattice dynamical calculation and the ultrasonic experiments establishes that the potential model provides a reasonable description of the elastic behaviour of this molecular crystal. The compression estimated from the Murnaghan equation of state agrees well with that estimated theoretically from the lattice dynamic calculations. The Debye temperature determined from the ultrasonic-wave velocities is 187.5 ± 2 K. Ultrasonic-wave velocities are linear up to about 100°C; the crystals do not undergo the transition to a monoclinic (β) phase which can take place at 95°C. There is no indication of softening of the long-wavelength acoustic phonon modes. Vibrational anharmonicity is discussed in terms of the long-wavelength acoustic-mode Grüneisen parameters obtained from the generalized Grüneisen theory in the quasiharmonic approximation. The mean high-temperature long-wavelength acoustic-mode Grüneisen parameter γ̄ el H (= 2.72) is much larger than the room-temperature thermal Grüneisen parameter γ th (=0.54). In this molecular crystal the intermolecular volume is much more compressible than the intramolecular volume; the mode Grüneisen parameters for purely internal vibrational modes are small, whereas those for the external modes are large. At room temper­ature all phonon modes contribute so that γ th is made small by the small internal-mode contributions. In contrast, the weak, strongly pressure dependent intermolecular forces dominate the zone-centre acoustic modes and lead to a large mean Grüneisen parameter γ̄ el H .