Phys Chem Minerals (2009) 36:439–446 DOI 10.1007/s00269-009-0290-6 123 ORIGINAL PAPER InXuence of high-pressure processing on the structure and memory eVect of synthetic layered double hydroxides Jaqueline C. Rodrigues · Tania Maria Haas Costa · Márcia R. Gallas · Celso C. Moro Received: 9 May 2008 / Accepted: 16 January 2009 / Published online: 5 February 2009  Springer-Verlag 2009 Abstract We investigate the structural evolution of synthetic layered double hydroxides (LDH) samples, processed at room temperature and high-pressure (up to 7.7 GPa) in a toroidal chamber with two pressure-transmit- ting media, lead and graphite, using X-ray diVraction, ther- mogravimetry and N 2 -adsorption isotherms techniques. The X-ray patterns of compacted samples show a decrease in the peak intensities. For both pressure-transmitting media, our samples revealed a reduction of the basal d-spacing for the (003) plane when processed at 7.7 GPa. The expected high-pressure-induced amorphization was not observed. Surprisingly, we Wnd high-pressure processing to have a strong inXuence on the memory eVect of the LDH, due essentially to the reduction of the surface area and pore closing. Even when immersed in water, our samples did not recover the LDH structure, when either calcined at 450°C and compacted at 7.7 GPa, or calcined at 700°C and imme- diately compacted at 2.5 and 7.7 GPa. Keywords Synthetic LDH · High pressure · Memory eVect · Double hydroxides Introduction Anionic clays, also called layered double hydroxides (LDH) or hydrotalcite, have been studied since the begin- ning of the 20th century because of their mineralogical interest (Crepaldi and Valim 1998). Later on, in the 80s, these clays were explored as promising materials for tech- nological applications such as, for example, precursors to other materials like cements, as catalyst and catalyst sup- ports, as anion-exchangers and adsorbents, and in chemical process industry (Prado 2003; Sabbar et al. 2007; You et al. 2002; Ferreira et al. 2007). A typical hydrotalcite (HT) is the naturally occurring hydromagnesite (i.e., Mg6Al2(OH)16CO3·4H2O), which consists of positively charged brucite-type octahedral sheets having carbonate anions and water molecules between them. The positive charges arise from substitutions of Mg 2 + by Al 3 + cations (Bellotto et al. 1996). The interla- melar domain consists of water and anions arranged in a disordered way, connected by van der Waals and H bridges interactions. Brucite-type octahedral sheets may be stacked in diVerent ways, generating a large number of polytypes as described by Brookin and Drits 1993. Synthetic HT com- monly crystallizes in a triple-layer cell with a rhombohedral symmetry (3R) and two types of stacking, called 3R 1 and 3R 2 (Thomas et al. 2004). Layered double hydroxide clays loose hydration water and CO 2 , when calcined at temperatures higher than 400°C, forming a double oxide-hydroxide (Velu et al. 1999). How- ever, when exposed to air, these calcined LDH clays have the ability of recovering their original structure, re-absorb- ing water and CO 2 , a property called memory eVect. When calcined at temperatures higher than 900°C, the decomposi- tion becomes irreversible, forming stable phases with spinel-like structures. Based on the memory eVect, calcined J. C. Rodrigues · T. M. H. Costa (&) · C. C. Moro Instituto de Química, Universidade Federal do Rio Grande do Sul, P.O. Box 15003, Porto Alegre (RS) 91501-970, Brazil e-mail: taniaha@iq.ufrgs.br M. R. Gallas Instituto de Física, Universidade Federal do Rio Grande do Sul, P.O. Box 15051, Porto Alegre (RS) 91501-970, Brazil