Dear all,
The downstream end of this thread seems to have drifted into learned
considerations of spelling, so I am getting back to this early reply.
I am surprised that nobody has mentioned the role of the wavelength in
all this: there is no way that one can directly link the first four planes
in a Nickel crystal to a fixed set of 2theta values. The values you quote,
Kianoush, must have been observed for a certain wavelength, but they would
be different for another wavelength. So if you want one of the powder rings
to come out at a 2theta of 45 degrees, adjust the wavelength accordingly so
that Bragg's law be satisfied for the spacing between the corresponding
planes.
There also seems to be a confusion in the last question (unless I have
completely misunderstood it) about the orientation of a crystal and the
Bragg angle at which it will contribute to the ring pattern of the powder it
belongs to. If there is a crystal oriented with some if its planes at 45
degrees from the X-ray beam, that will simply determine where on each ring
its diffraction spots will contribute: it will have no effect on the Bragg
angles of those spots, that depend purely on the internal spacings between
atoms within the crystal, not on the orientation of the crystal. At the same
wavelength at which you quote the 2theta values for those four rings, the
crystal at 45 degrees from the beam will still have its diffraction spots
contribute to the rings at 44, 52, 76 and 93 degrees.
Again, forgive me if I have completely misunderstood the initial
question.
With best wishes,
Gerard.
--
On Tue, Jul 08, 2014 at 04:13:59PM -0400, Edward A. Berry wrote:
> The plane will scatter, and all atoms in the plane will scatter in phase
> if angle of incidence equals angle of reflection. this is how a mirror
> reflects. Furthermore all the parallel planes will also reflect at this angle.
> Trouble is the beams scattered from the different parallel planes are
> systematically out of phase with each other unless Bragg's law is met
> for that set of planes, so interference is destructive and adds up to nothing.
> At least that's how I understand it,
> eab
>
>
>
>
> On 07/08/2014 03:53 PM, Kianoush Sadre-Bazzaz wrote:
> >Hi
> >
> >If a sample of powder crystal (say Nickel) is shot with monochromatic
x-rays, one will observe reflections from planes that satisfy Bragg's Law.
For Ni the first four planes are (111, 200, 202, 311) with 2theta (44, 52,
76, 93 degrees) respectively.
> >
> > Why doesn't one observe a reflection at, say, 45 degrees? There will be
a grain oriented in the powder such that x-rays reflect at 45 degrees and so
forth. I would expect a continuum of reflections...
> >
> >thanks for the insight.
> >
> >Kianoush
> >
--
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