research papers 520 doi:10.1107/S1600576715002976 J. Appl. Cryst. (2015). 48, 520–527 Journal of Applied Crystallography ISSN 1600-5767 Received 22 July 2014 Accepted 11 February 2015 # 2015 International Union of Crystallography Determination of the melting and freezing temperatures of Pb nanoparticles embedded in a PbO–B 2 O 3 –SnO 2 glass by using only the SAXS method G. Kellermann, a * A. Gorgeski, a A. F. Craievich b and L. A. Montoro c a Departamento de Fı ´sica, Universidade Federal do Parana ´, Curitiba, PR, Brazil, b Instituto de Fı ´sica, Universidade de Sa ˜o Paulo, SP, Brazil, and c Departamento de Quı ´mica, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil. Correspondence e-mail: keller@fisica.ufpr.br Melting and freezing of metallic nanoparticles embedded in glass matrices usually occur at temperatures lower than for the same metal in the bulk state. In situ small-angle X-ray scattering (SAXS) measurements using a synchrotron beamline and a specially designed high-temperature chamber allowed the determination of the temperature dependence of the SAXS intensity produced by a dilute and nearly monodisperse set of spherical Pb nanoparticles, with an average radius hRi = 16.1 nm, embedded in a homogeneous lead–borate oxide glass. The temperature dependences of the nanoparticle volume V(T) and nanoparticle radius of gyration R g (T) derived from SAXS results exhibit clear discontinuities during the cooling and during the heating processes, thus allowing for precise determinations of the melting and freezing temperatures of the studied Pb nanoparticles. Additional features observed in both V(T) and R g (T) curves showed that during the heating cycle the frozen Pb nanoparticles suffer a transition to a more compact phase at 433 K before melting at 580 K. The results of this work demonstrate that the melting and freezing temperatures of nanoparticles in a very diluted state – for which the X-ray diffraction technique is not sensitive enough – can be precisely determined by applying only the SAXS method. 1. Introduction The formation of metallic and semiconductor nanoparticles embedded in glass matrices has attracted the attention of many scientists (Gonella & Mazzoldi, 2000; Craievich et al., 2002; Nogami & Abe, 1994; Yang et al., 2005; Hamanaka & Nakamura, 1999). The reasons for this are related to inter- esting basic aspects of the properties of nanostructured materials, which often exhibit unusual features, and also to the fact that they are potential candidates for the development of new optoelectronic devices (Mazzoldi et al., 1996; Pavesi et al., 2000). The first step for an adequate understanding of the physical properties of nanomaterials composed of a homogeneous matrix in which isolated nanoparticles are embedded is to properly characterize the relevant structural features of the nanoparticles, namely the shape, atomic structure and size distribution. Several techniques are commonly used to experimentally determine the liquid-to-solid (freezing) and solid-to-liquid (melting) transitions of nanoparticles embedded in homo- geneous matrices or supported on substrates (Takagi, 1954; Kofman et al., 1990; Zhao et al. , 2001, and references therein; Kellermann & Craievich, 2002, 2008; Itoigawa et al., 1997; Lopeandia & Rodriguez-Viejo, 2007; Valov & Leiman, 1997; Cheyssac et al., 1988). Some of these techniques, such as X-ray diffraction (XRD) (Kellermann & Craievich, 2002) and elec- tron diffraction (Takagi, 1954), are sensitive to the atomic arrangement inside the nanoparticles, thus allowing one to distinguish between crystalline and liquid or amorphous (disordered) structures. Others often applied are based on a combination of transmission electron microscopy (TEM) and thermal (Itoigawa et al., 1997; Lopeandia & Rodriguez-Viejo, 2007) or optical (Valov & Leiman, 1997; Cheyssac et al., 1988) analysis of the studied materials during heating and cooling processes. In a previous investigation the melting and freezing of Bi nanocrystals embedded in glass were investigated by simul- taneous use of small-angle X-ray scattering (SAXS) and XRD (Kellermann & Craievich, 2008). The correlation observed in the behaviors of XRD and SAXS patterns showed that the measurement of SAXS intensity curves as a function of temperature is a useful method to investigate the phase transitions of nanoparticles embedded in glass matrices. It was established in this study that discontinuities in the integrated SAXS intensity versus temperature curves at the crystal-to- liquid and liquid-to-crystal transitions – a consequence of the