were made by the Norwegian-born
US chemist Lars Onsager (1903–76).Debye–Hückel theoryA theory to
explain the nonideal behaviour of
electrolytes, published in 1923 by
Peter *Debye and Erich Hückel
(1896–1980). It assumes that elec-
trolytes in solution are fully dissoci-
ated and that nonideal behaviour
arises because of electrostatic interac-
tions between the ions. The theory
shows how to calculate the extra free
energy per ion resulting from such
interactions, and consequently the
activity coefÜcient. It gives a good de-
scription of nonideal electrolyte be-
haviour for very dilute solutions, but
cannot be used for more concen-
trated electrolytes.Debye–Scherrer method A tech-
nique used in *X-ray diffraction in
which a crystal in powder form is
Üxed to a thinÜbre or thin silica
tube, which is then rotated in the
path of a beam of monochromatic X-
rays. A circular diffraction ring,
called the Debye–Scherrer ring, con-
centric with the undeÛected beam
is formed. The diffraction pattern is
recorded on a cylindricalÜlm, which
has its axis parallel to the axis of ro-
tation of the material. The Debye–
Scherrer method is used to obtain in-
formation about the material. The
grains of the powdered crystal must
be much larger than the atomic di-
mensions in order for them to dif-
fract X-rays.
Debye temperature See debye
theory of specific heat.
Debye theory of speciÜc heatA
theory of the speciÜc heat capacity of
solids put forward by Peter *Debye in
1912, in which it was assumed that
the speciÜc heat is a consequence of
the vibrations of the atoms of the lat-
tice of the solid. In contrast to the
*Einstein theory of speciÜc heat,which assumes that each atom has
the same vibrational frequency,
Debye postulated that there is a con-
tinuous range of frequencies that
cuts off at a maximum frequency νD,
which is characteristic of a particular
solid. The theory leads to the conclu-
sion that the speciÜc heat capacity of
solids is proportional to T^3 , where T
is the thermodynamic temperature.
This result is in very good agreement
with experiment at low tempera-
tures.
A key quantity in this theory is the
Debye temperature, θD, deÜned by θD=
hνDk, where h is the Planck constant
and k is the Boltzmann constant. The
Debye temperature is characteristic
of a particular solid. For example, the
Debye temperature of sodium is 150
K and the Debye temperature of cop-
per is 315 K.Debye–Waller factor A quantity
that characterizes the effect of lattice
vibrations on the scattering intensity
in X-ray diffraction by crystals. The
existence of the Debye–Waller factor
was pointed out and calculated by
Peter *Debye in 1913–1914 and Ivar
Waller in 1923–1925. Because the
amplitude of lattice vibrations in-
creases with temperature, it was
thought in the very early days of X-
ray diffraction studies that the dif-
fraction pattern would disappear at
high temperatures. The work of
Debye and Waller showed that the
lattice vibrations at higher tempera-
tures reduce the intensities of the dif-
fracted radiation but do not destroy
the diffraction pattern altogether.deca-Symbol da. A preÜx used in
the metric system to denote ten
times. For example, 10 coulombs = 1
decacoulomb (daC).decahydrateA crystalline hydrate
containing ten moles of water per
mole of compound.Debye–Hückel theory 164d