Rendering proper materials I am a beginner at rendering! I am trying to render a very simple object, a sphere with a single material added with the principled shader. I would like to make a high-reflecting metallic sphere.

I don't understand why I see something similar to what I want in the LookDev Method (I use Blender 2.80), but something totally different (i.e. a diffusive and very dark sphere) when I render the scene. I attached the pictures of both the LookDev Method and the rendered image.

I think that the materials settings are more or less fine (cause I see it properly in the LookDev Method), and I have tried both Cycles and Eevee as render engines, including the Ambient Occlusion and employing an HDR environmental texture.

Could anybody suggest me which rendering settings are key to obtain the desired result? 

Rendered vs LookDev display
LookDev mode is intended to give a basic approximation of how the material will behave. Those reflections? They're a built-in environment texture so that we can see the reflections of the surface. Rendering is a different process tho, and uses separate lighting. 

Lighting is a pretty complex topic, but one good way to get reliable results is to use an HDRI - like lookdev mode does. Hdris can easily provide good lighting and realistic reflections. This is perfect in your case too, because the reason that the rendered sphere looks flat is because it only has a uniform color to reflect! The best free high-quality HDRI source is hdrihaven.com To learn about how to use them in your scene, see this How can I render an object with an hdr background?

I hope this helps!