Transformation of nanocrystalline Mg(OH) 2 to MgO under dynamic vacuum in a temperature range of 570-770 K was studied by TGA-GCMS, FTIR, HREM, XRD, helium pycnometry, and nitrogen adsorption at 77 K. Several processes are shown to occur during transformation including: (a) release of compounds chemisorbed during preparation of nanocrystalline Mg(OH) 2 simultaneously with hydroxide-oxide transformation; (b) fragmentation of nanoparticles at temperatures lower than 670 K; and (c) re-crystallization and sintering of nanoparticles, whose mean size increases and relative number decreases rapidly with temperature above 670 K. A new phenomenon of decrease of the interlayer distance in MgO crystallites with increase of the nanoparticle size was observed. IntroductionThe transformation of Mg(OH) 2 prepared from different precursors to the corresponding MgO during thermal treatment was studied extensively in the past (see, e.g., ref 1 and the references within). Conventionally prepared magnesium hydroxide has the structure of hexagonal, smooth crystalline platelets with a size of 10-100 nm. The specific surface area is typically about 10-70 m 2 /g and the volume of pores is in the range of 0.1-0.7 cm 3 /g. Decomposition at 500-600 K results in pseudomorphous transformation of each single platelet, thus the newly appeared numerous MgO crystals of much smaller size (5-20 nm) form a porous aggregate of the same volume and shape the original platelet had. Thus, porous MgO aggregates appear from nonporous Mg(OH) 2 platelets. Increase of a treatment temperature results in sintering of MgO crystals (decrease in their relative number and increase in size).

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