Effect of Water Species on the Phonon Modes in Orthorhombic Y₂(MoO₄)₃ Revealed by Raman Spectroscopy

Abstract

Negative thermal expansion material of yttrium molybdate has been synthesized by rapid solidification with a CO 2 laser. It crystallized in the orthorhombic phase with unique microstructures consisting of nanoparticles or nanodendrites with grain sizes around 20-30 nm. Raman spectroscopic and XRD analyses reveal that the presence of water species hinders not only the rocking motion but every type of motion of the corner shared polyhedra. Because of the presence of water species, degeneracy of the Raman bands happens for symmetric stretching, asymmetric stretching, and the bending modes, and the lattice, translational, and librational modes cannot be observed. The release of water species below 403 K (388 K for the newly prepared sample or 383 K in the second temperature cycle) has little whereas that above this temperature has great effect on the stretching and bending vibrations and also the librational and translational modes. This indicates that the water species which are released within the first step do not interact with the MoO 4 tetrahedra in the Y 2 -Mo 3 O 12 framework while those released in the second step have a direct interaction. It is suggested that the translational and librational motions are closely coupled with stretching and bending vibrations. They together give rise to the negative thermal expansion.

Raman spectroscopy study of Na2MoO4·2H2O and Na2MoO4 under hydrostatic pressure

“…In other words, the molecule of water is linked to the corners of the tetrahedra impeding the bending vibration, according to the information by Liang et al [18] …”

Relationship between hygroscopicity reduction and morphology evolution of Y2Mo3O12 doped with (LiMg)3+

“…However, NTE materials have not utilized widely in practical applications due to several disadvantages, such as heavy hygroscopicity, high phase transition temperature, metastable structure [7][8][9][10], etc.…”

“…The investigations about the hygroscopicity and its reduction of A 2 Mo 3 O 12 with theoretical simulation and experimental method [9,[20][21][22] pointed out that the hygroscopicity affecting the NTE property of A 2 Mo 3 O 12 family is related to the "A" cations size. A 2 Mo 3 O 12 with A 3+ from Lu 3+ (85 pm) to Y 3+ (90 pm) crystallizes in orthorhombic structure with heavy hygroscopicity at room temperature [20,23,24].…”

“…The orthorhombic structure consists of corner sharing AO 6 octahedra and MoO 4 tetrahedra with A-O-Mo linkages in the open framework structure [6,17,25], which can accommodate for transverse thermal vibrations of oxygen bridge atoms responsible for NTE. Whereas the openings in the network with larger A 3+ cations close to that of Y 3+ (90 pm) become large enough to admit water molecules that inhibit the flexibility as well as the internal vibrations of the polyhedra which are essential for NTE [9]. Partial substitution of Y 3 + with smaller trivalent cations, such as Fe 3+ , Al 3+ , Cr 3+ , etc.…”

Rapid synthesis of La0.9Sr0.1Ga0.8Mg0.2O3− electrolyte by a CO2 laser and its electric properties for intermediate temperature solid state oxide full cells

“…The synthesis of pure bulk materials with laser rapid solidification (LRS) is a relatively new technique [21][22][23][24][25]. In this paper, laser rapid solidification was employed to synthesize LSGM electrolyte and the microstructure and electrical properties of the samples synthesized by laser were studied and compared to the samples synthesized by solid state reactions.…”