Research Article Received: 15 December 2009 Accepted: 4 May 2010 Published online in Wiley Interscience: (www.interscience.wiley.com) DOI 10.1002/jrs.2720 Pressure-induced anomalous phase transformation in nano-crystalline dysprosium sesquioxide Nita Dilawar Sharma, a* Jasveer Singh, a Sugandha Dogra, a D. Varandani, a Himanshu Kumar Poswal, b S. M. Sharma b and A. K. Bandyopadhyay a The phase transformation in nano-crystalline dysprosium sesquioxide (Dy 2 O 3 ) under high pressures is investigated using in situ Raman spectroscopy. The material at ambient was found to be cubic in structure using X-ray diffraction (XRD) and Raman spectroscopy, while atomic force microscope (AFM) showed the nano-crystalline nature of the material which was further confirmed using XRD. Under ambient conditions the Raman spectrum showed a predominant cubic phase peak at 374 cm -1 , identified as F g mode. With increase in the applied pressure this band steadily shifts to higher wavenumbers. However, around a pressure of about 14.6 GPa, another broad band is seen to be developing around 530 cm -1 which splits into two distinct peaks as the pressure is further increased. In addition, the cubic phase peak also starts losing intensity significantly, and above a pressure of 17.81 GPa this peak almost completely disappears and is replaced by two strong peaks at about 517 and 553 cm -1 . These peaks have been identified as occurring due to the development of hexagonal phase at the expense of cubic phase. Further increase in pressure up to about 25.5 GPa does not lead to any new peaks apart from slight shifting of the hexagonal phase peaks to higher wavenumbers. With release of the applied pressure, these peaks shift to lower wavenumbers and lose their doublet nature. However, the starting cubic phase is not recovered at total release but rather ends up in monoclinic structure. The factors contributing to this anomalous phase evolution would be discussed in detail. Copyright c  2010 John Wiley & Sons, Ltd. Keywords: high pressure; Raman spectroscopy; phase transitions; Dy 2 O 3 ; XRD; AFM Introduction Rare earth sesquioxides are considered vital in the process of ceramics as additives for low temperature sintering, grain growth inhibitors, as phase stabilizers and for applications in nuclear engineering. Dysprosium sesquioxide is also highly magnetic. Dy 2 O 3 has many applications in high performance luminescent devices, optical fibers, dopants for fluorescent materials, halide lamps etc. [1] High purity Dy 2 O 3 is also used as antireflection coating in photoelectric devices. Dysprosium oxide thin films also find applications in optical devices and dielectric materials. Cubic Dy 2 O 3 also has a strong potential for applications in manufacture of superconductors and also used as a catalyst. [2,3] In the form of nano-structures, it holds potential as a highly functionalized material as a result of both enhanced surface area and quantum confinement effects. Potential applications of Dy 2 O 3 in nano-powder form include its uses as dopants for fluorescent materials, as glass material with a Faraday rotation effect for optical and laser-based devices, as magneto-optical recording materials, and materials with a large magnetostriction. It can also be used for the measurement of neutron energy spectrum, as nuclear reaction control rods and as neutron absorbents. [4] The crystallographic data and polymorphism of rare earth sesquioxides were first reviewed by Brauer [5] and updated by Haire and Eyring. [6] The rare earth sesquioxides at ambient temperature and pressure are known to exist in three structural modifications, and these were first investigated systematically by Goldschmidt and his coworkers in 1925. [7] These three phases are designated as A, B and C, which correspond to hexagonal (in most cases space group P3m1), monoclinic (in most cases space group C2/m) and cubic phase (in most cases space group Ia3), respectively. The cubic phase unit cell has 16 molecules per unit cell and the resulting structure has 24 Ln 3+ ions on sites with C3 3i (S6) symmetry. [8 – 10] The stability of Ln 2 O 3 depends on radius ratio of cation and anion at ambient pressure and temperature. At high temperatures, two additional polymorphs occur, which are H-type hexagonal with space group P6 3 /mmc and X-type, Im-3m cubic structure. Because C → B and B → A phase transitions are accompanied by volume contraction, the application of pressure increases the stability of B and A structures relative to C and B modification respectively. With increase in temperature/pressure the phase transitions usually follow the sequence C → B → A for most of Ln 2 O 3 . Hydrostatic pressure and shock wave experiments have been performed in a few rare earth sesquioxides using Raman scattering and X-ray diffraction (XRD). [8,11 – 13] Recently, the authors have reported cubic to hexagonal transformation in nano-crystalline Gd 2 O 3 and Y 2 O 3 . [14] This transformation had been reported for ∗ Correspondence to: Nita Dilawar Sharma, Pressure & Vacuum Standards, National Physical Laboratory, New Delhi 110012, India. E-mail: ndilawar@mail.nplindia.org a Pressure & Vacuum Standards, National Physical Laboratory, New Delhi 110012, India b High Pressure Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India J. Raman Spectrosc. (2010) Copyright c  2010 John Wiley & Sons, Ltd.